I also endorse the replacement panel.

I have 2 working, 1 problem child / donor unit-- I found that upgrading the opamp on the output board really improved the sound of these units after doing the well documented grounding change (which I can't remember but involved cutting a wire I believe).
 
I can run them at full volume without worrying about hiss, bass sounds clearer and more like you would want from a synth...  I can't remember the exact parts that I ordered but they were in the $3-$15 range, likely something from burr-brown (maybe a newer TI excaliber TL207x) that was well regrading in the DIY audio community.  The change wasn't subtle.  Next time I have a unit open I'll take a closer look.  It isn't a difficult mod -- remove old chip, add socket, add new chip.   I'm sure someone industrious could do a drop-in board instead...

-Ben