John:
In the KURZWEIL line of digital pianos (maybe some of the portable KB too) I have seen this language in the promo material:

"Records and plays back auto-accompaniment performances. Can play Standard MIDI Files and can read and convert disks from Yamaha PianoSoft and Disk Orchestra Libraries, plus PianoDisc Music Library."

Perhaps you have a contact who owns or sells Kurzweil models, and couple see if this might work:

Load floppy into Kurz, copy to hard disk, copy back to (another) floppy as MIDI file (which you MIGHT be able to then rename to *.STY and use in Yamaha boards).

OR -- read Yamaha files into Kurz hard disk one by one -- place new DOS-format diskette into Kurz floppy drive -- copy all right back without conversion. At least then YOUR computer should be able to see and move the files around.

But first option might work best all around.

Worth a try if you really want to keep and use these proprietary format files.