Hohner Pianet L simple preamp replacement

[AlfredoMeister]

I have an old Hohner Pianet L, one of the ones with built in speakers and a little solid state amp circuit. It's got terrible 60hz hum (already grounded the key shafts, no dice), and a pretty low output. Somebody modified it in the past and added a line out, seemingly tapping from the preamp. The whole circuit is on ancient tagboard and lord knows what's wrong with it, so I was wondering if there are any simple preamp kits or prebuilt boards that I replace the circuit in my Pianet with to provide a clean line level out? thanks.
(maybe a Pianet N or Combo board?).