Looking for help modifying a 1980s synth keyboard

Urbana IL

Description

I have a Yamaha PSR-27 I want to load a custom voice onto. I couldn't find any information online on how this might be done, but certainly it must be possible. If you know how -- or even just feel like you know enough to find out -- please contact me. The same keyboard also has a few unresponsive keys, so I would also appreciate help repairing those.