will it read the MIDI files I modified for the MX, or would I have to do them all over again a third time?

You will likely have to do a similar thing with any other model you purchase. Modify the instrument selection... the Recording will transfer, but you are going to be selecting from a much larger Sound Set — so you’re going to want to take the time. New Effects, per Part EQ available, etc., etc., etc.

The MX88 will have the exact same soundset, and therefore will work right away with your data prepared for the MX61. The MOXF has a different Voice set from the MX-series. And the MODX has yet a different configuration, altogether.

The MX series 166MB Wave ROM
The MOXF series 741MB Wave ROM
The MODX series 5.67GB Wave ROM

I looked at the MODX locally, but couldn't get it to play these MIDI files. It would only load the first 4 MIDI channels. WTH?

Without knowledge of the MODX its not that surprising that you could not get it to work right away. You’d need to know a few things ...It would take at least a little knowledge of how the architecture differs. Then you would have been successful...
Did you place it on the Performance “Multi/GM” before playing your file? No? If not, you would have inherited the current number of Parts. If you start with a 4 Part setup... it assumes you did so on purpose.
That could be why it only played the first four channels, you only had four Parts. Selecting “Multi/GM” as a starting point would setup the MODX with 16 individual instruments, one per Part (just like your MX Performance). Had you done that, 16 Parts would play.

This is accomplished slightly differently in the MOXF, as well. There, you would need to place it in what the MOXF-Series refers to as a “Song Mixing“ or “Pattern Mixing” program... again this puts the MOXF in the familiar layout like your MX... 16 Parts, one instrument in each.

If you place the MOXF in Performance mode you would only be able to play 4 Parts simultaneously. The MOXF Performance is 4 Parts (similar to the MX’s Part 1/2 and how they create Layers and Splits)... the MOXF Performance lets you create Layers and Splits using 4 Parts on the keyboard at once.
You need to go to a specific MOXF Mode (Sequencer Mode) to work with a 16 channel sequencer.
The MODX doubles this capability yet again, allowing you to assign 8 Parts in your complex Layer and Splits — you do so by linking Parts 1-8 with KBD CTRL (Keyboard Control).

The MX has 2 Arps, the MOXF has 4 Arps, the MODX has 8 Arps
The MX layers 2 Parts, the MOXF layers 4 Parts, the MODX layers 8 Parts
The MX is 16 Part multi-timbral, the MOXF is 16 Part multi-timbral, the MODX is 16 Part multi-timbral.
They all can playback (respond to) your .mid file.
Only the MX88 uses the same exact layout as your MX61.
MOXF8 and the MODX8 are both just over 30lbs
Both MOXF and MODX can load your .mid file...(They both have internal Sequencers)... you will have tons of new sounds to choose from, so worst case, you’ll have to select a better sound from the larger sound set. grin. But at least you will not have to replay all of the TRACKS.

Hope that helps. They both can do the job, it’s a matter of price point and *just how much fun you want to have*!!!! Let us know when you decide! Anticipate reworking the file ‘yet again’ but trust me, with the advances in what you get to do to the Tracks is going to be fun. You’ll pretty much be able to find the instrument you originally used, but now you have Effects and EQ that you can customize per sound. As the song takes shape you’ll feel that improvement. The hard part (playing the tracks) is done. Improving and mixing is what you’ll be doing now.

will it read the MIDI files I modified for the MX, or would I have to do them all over again a third time?

You will likely have to do a similar thing with any other model you purchase. Modify the instrument selection... the Recording will transfer, but you are going to be selecting from a much larger Sound Set — so you’re going to want to take the time. New Effects, per Part EQ available, etc., etc., etc.

The MX88 will have the exact same soundset, and therefore will work right away with your data prepared for the MX61. The MOXF has a different Voice set from the MX-series. And the MODX has yet a different configuration, altogether.

The MX series 166MB Wave ROM
The MOXF series 741MB Wave ROM
The MODX series 5.67GB Wave ROM

I looked at the MODX locally, but couldn't get it to play these MIDI files. It would only load the first 4 MIDI channels. WTH?

Without knowledge of the MODX its not that surprising that you could not get it to work right away. You’d need to know a few things ...It would take at least a little knowledge of how the architecture differs. Then you would have been successful...
Did you place it on the Performance “Multi/GM” before playing your file? No? If not, you would have inherited the current number of Parts. If you start with a 4 Part setup... it assumes you did so on purpose.
That could be why it only played the first four channels, you only had four Parts. Selecting “Multi/GM” as a starting point would setup the MODX with 16 individual instruments, one per Part (just like your MX Performance). Had you done that, 16 Parts would play.

This is accomplished slightly differently in the MOXF, as well. There, you would need to place it in what the MOXF-Series refers to as a “Song Mixing“ or “Pattern Mixing” program... again this puts the MOXF in the familiar layout like your MX... 16 Parts, one instrument in each.

If you place the MOXF in Performance mode you would only be able to play 4 Parts simultaneously. The MOXF Performance is 4 Parts (similar to the MX’s Part 1/2 and how they create Layers and Splits)... the MOXF Performance lets you create Layers and Splits using 4 Parts on the keyboard at once.
You need to go to a specific MOXF Mode (Sequencer Mode) to work with a 16 channel sequencer.
The MODX doubles this capability yet again, allowing you to assign 8 Parts in your complex Layer and Splits — you do so by linking Parts 1-8 with KBD CTRL (Keyboard Control).

The MX has 2 Arps, the MOXF has 4 Arps, the MODX has 8 Arps
The MX layers 2 Parts, the MOXF layers 4 Parts, the MODX layers 8 Parts
The MX is 16 Part multi-timbral, the MOXF is 16 Part multi-timbral, the MODX is 16 Part multi-timbral.
They all can playback (respond to) your .mid file.
Only the MX88 uses the same exact layout as your MX61.
MOXF8 and the MODX8 are both just over 30lbs
Both MOXF and MODX can load your .mid file...(They both have internal Sequencers)... you will have tons of new sounds to choose from, so worst case, you’ll have to select a better sound from the larger sound set. grin. But at least you will not have to replay all of the TRACKS.

Hope that helps. They both can do the job, it’s a matter of price point and *just how much fun you want to have*!!!! Let us know when you decide! Anticipate reworking the file ‘yet again’ but trust me, with the advances in what you get to do to the Tracks is going to be fun. You’ll pretty much be able to find the instrument you originally used, but now you have Effects and EQ that you can customize per sound. As the song takes shape you’ll feel that improvement. The hard part (playing the tracks) is done. Improving and mixing is what you’ll be doing now.

Bad Mister Phil, I very much appreciate these answers! That clears up everything. I couldn't have dreamed of a better response.

I don't know if you remember me. We crossed paths many years ago at NAMM shows when I was the product guy at Casio from the late 80s and 90s.
I even did some demos and trade shows for Yamaha after I left Casio.
I am finally now gravitating toward Yamaha gear all the time, which I should have done years ago.

Based on what you told me, I am going to take a serious look now at the MODX, although I guess the MOXF is still pretty darn good.
Thanks!
Best,
Ed