DIY replacement belts
You can try replacing the drive belt with a similar sized elastic band for a temporary fix, but it is much better to find a replacement of the right dimensions so that it is neither too tight or too loose (both conditions are likely to prevent the drive from working reliably). Unlike many cassette deck drive belts, the belt used in the SY99 drive is flat instead of square, and is more like the belt used in belt driven record decks of the same vintage.

If you do use an exastic band for a temporary fix, then you should first verify the correct operation of the drive by formatting a blank floppy disk, then saving a song to it, and then checking that the song can be loaded... Only when this works correctly and reliably should you use the floppy drive.