My Guess...

it was the KaPro CS80 Library.

For the performances that don't work.... edit them if possible and write down all the Element names from every part.

It is possible you will find these in the KaPro library, and can replace them once this Library is installed.

The KaPro CS80 download is here:
https://shop.usa.yamaha.com/en/c/downloadables/sound-expansion-library/synth-voice-libraries

Thanks, yes it was the KAPro library. When I navigated to the link, I could see the previous 'purchase' with download links. Hopefully once these are loaded the performances can be reconstructed.

To compliment the info what Anthony and Dragos already already provided, here is a breakdown of what most likely have happened.

I suspect you loaded the KAPro CS80 library at one point and then only saved the selected user performances to your user area instead if using the "library import" function.

Doing this will only save the performance and link the used waveforms instead of also saving them to the user area.

You could recover from this by loading the library again but it's tricky as you are not able to select which library slot to use amd this is key for making it work.

Anthonys suggestion will help mitigate this but I would try this instead.

If you already know which performances are broken then replacing them with the equivalent from the library this time using the library import will fix them.

If you don't really know then you could install the demo version of John Melas Waveform Editor for Montage and open the MODX .x8a backup.

There you can see which library slot was used for each library.

Too my knowledge it seem like the libraries are loaded into the first available slot.

So if you find the cs80 library to have been loaded into slot 3 make sure to load two other libraries before loading it to get it properly aligned.

Hope this make sense..

I presume that you have the Library for the CS80 loaded into the Montage that you had Saved from the borrowed MODX, but the Performances in the User area are not linking correctly to it, correct..!?
If so, then when you loaded the User & Library files into the Montage, did you Load them in the exact same order (Slots) that they were on the MODX?

In order for the waveforms in the User memory Performances to line up/link with the libraries, the libraries must be in the same specific slots that they were on the MODX.

If you don't know the order that they were originally loaded on your MODX that was stolen, then you can find out which slots each Library was in (on your MODX) by checking a Performance in User memory that was linked to a specific Library, and make note of the Bank Select MSB & LSB numbers ... then repeat this on one User memory Performance for each Library. *You may at this point only need to load the CS80 library back into the correct slot.
It's the LSB number that really tells which slot that the library was/should be in!

So for Performances in the User area that have waveforms which are linked to a specific library, to find out which slot the library was in (LSB), open the Performance (Home), touch the Performance's name, and select the "Property" option from the pop-up menu on the left - this screen will tell you the slot by number for the library by looking at the LSB number.

Repeat this on at least one Performance (for each of the different libraries) that points back to the various libraries. So in my case, I keep record of the LSB (& MSB) numbers for each library that I have loaded that are "long term" libraries that I plan to keep permanently for the specific User memory Performances that I save (see list below). Here are the LSB numbers that show the order of my libraries. So in the event that I need to load the Libraries back in, I have to make sure to start with the "MyDX7II-Lib" Library 1st and end with the "Organimation" library ... basically in the order you see below:

(Note that this may not be the order you see them in when you go to Utility > Contents > Data Utility > Library; However as long as they are loaded with the correct LSB slot number, they should link properly in the User Performances)

1 - MyDX7II-Lib (MSB 63 - LSB 40)
2 - AG MONTAGE (MSB 63 - LSB 45)
3 - Synthogy Ivory C7 (MSB 63 - LSB 50)
4 - Bosendorfer Grand (MSB 63 - LSB 55)
5 - Organimation (MSB 63 - LSB 60)

Then I would check each User Performance to ensure they are linking properly! If not, then you may possibly have to re-load the User File..!?
(Hopefully you haven't saved any new User area Performances or made any modifications. If you did save new/modified Performances, then you could back those specific ones up first using Montage Connect, then re-load the User File, and then Load those specific Performances back via Montage Connect.)

Regarding the CS80 library, if there is a library already loaded in the LSB number slot where it needs to go, you will need to Delete the specific Library that is in that slot, then Load the CS80 Library. If there are gaps between the LSB numbers for the Libraries, then you may need to load a temporary small library first. For example if the CS80 library needs to be in LSB 60, but it keeps loading into the first LSB 40 slot (or one of the slots in between LSB 40 & 60), then load a temporary library to fill slot LSB 40, then load the CS80 library.

Hope this helps.

Thanks for this really useful information. I did manage to load the CS80 library in. Luckily I had kept part of the original name in my edited patches so it was fairly easy to reconstruct what I needed.
Daryyl - Just to clarify - No, the CS80 library didn't come over from MODX either in export or import. So I didn't have the CS80 sounds anywhere. I only had access to the MODX for a limited time so going back & re-trying would have been difficult
The information re Library order is good to know. I'll make note of this on my instrument
Once again for anyone who needs to hear it - backup regularly! - As heart breaking as it is to lose an instrument, it makes it far worse when you lose the hours you've spent getting it sounding how you like it.