We moved your post to the Vintage Yamaha Synths Forum for items from the last century.

You've unfortunately waited until the floppy disk drive is gone almost 17 years now... yikes!
You can still find 3 1/2" replacement drives out there, but as is the way on rare items, they are probably move expensive than they were back in the 1990s. The V50 dates back to 1989... you will need to do some research, there maybe retrofit kits that can replace the floppy drive and offer you a more modern storage media.

But if you have tons of data on floppies, you'll need to transfer that data to something else. I read somewhere the future is pock marked with moving your important data from media to media about every seven years... it's time for stuff you have on DVDs to be moved to a newer storage media. The time to have converted 3.5" floppies was somewhere around 2005.

If you are in the USA call Customer Support, they are usually up on what options are available to replace floppy drives.

Just open the V50 (follow the steps in Service Manual), remove FDD, open it and replace the old tired belt. That's all. Don't forget to clean all around the wheels, it can be dirty from the old belt gum. Use isopropylalcohol.
Another possibility is to replace FDD with FDD emulator, like Gotek, Lotharek or similar. They use USB memory stick or cards.

And there are also adapters allowing using of standard PC 32 pin FDD. Check E-bay or FB DX group.