Welcome to the Forums! First off, this guitar forum does not get a lot of traffic, so you'll have to be patient and I know that's difficult when you need to repair something. I don't have a guitar background, so my offerings may or may not apply. I'm kind of surprised that Yamaha does not stock the circuit boards for those guitars, they are usually pretty good about parts for older keyboards. I know from other work that I've seen with electronics that the circuit boards rarely go bad but the components (such as capacitors, etc.) do go bad but can be replaced or you may also have bad solder joints to re-do. On one of the Hammond forums I'm on people are constantly "re-capping' or installing new capacitors in their Hammonds. A schematic of your board would likely prove to be very helpful if you want to repair it, I don't know if they are readily available anywhere.
DX7IIFD, SY77, SY99, Hammond C3, Steinway L, CP300, AW1600, etc.