I think it very much IS a custom display for Yamaha because of the obvious fact that it is not simply dot-matrix, but rather has hard-coded glyphs specifically for its purpose. How many standard displays out there have "UNISON"/"DUAL"/"ARPEGGIO"/etc. written on them? Think about things first.
OP, in what way is your display defective? If the backlight, most people (i.e. a few I've read) just stick a few LED lights in there, and there's probably a guide/descriptin you can read somewhere.