I own one that I'm currently working on restoring. It came to me non-functional and I now have it in a relatively playable state.
Things I have done so far: replaced all the capacitors, power supply board component upgrades, new headers on power supply and MPU boards, replaced burned out transistors and resistors, replaced burned out coils, rebuilt all flippers, added remote battery pack, added rectifier fuses, 40-pin connector replaced, all bulbs replaced with 47s, replaced chip sockets on sound board, new rubbers, new shooter sleeve, springs, tip.
Still a few issues: Sound board doesn't work at all, despite proper voltages, working speaker, working amp. One or more of the 5-bank targets isn't triggering the all-down series and it doesn't pop back up during gameplay.
Improvements still in progress: gradually replacing all headers, wiring harness contacts, and chip sockets