Hi Eric,
The Polysix is a pretty straightforward synth to repair. I've repaired several of them through the years, and they all suffered from these issues: Corrosion of tracks near the leaking NiCd battery and failing SSM 2044 ICs.
Changing the battery is easy, but repairing the tracks requires some soldering skills.
I had no problems finding brand new SSM 2044 ICs on the interweb.
Scott Rider "Old Crow" has some very useful info on his site on how to repair a Polysix. There's also a link to the svc manual:
Old Crow's Synth Shop: Korg Polysix Upgrade/Repair Overview
I hope this has encouraged you to repair it yourself. If any question arise in the process, please feel free to ask us here in the group. Many experienced persons here are willing to help.
BTW, in case you want to sell anyway... where is the synth located?