Modern op amps, filter caps, output coupling caps are a good place to start. If it has issues reading burned CD's most likely a dried out 33uf cap on the servo board. You can delete muting transistors, and look into non-oversampling. After that, you can look into an aftermarket clock, new TDA1541 decoupling caps, the list goes on and on. I have a mildly modified one with non-OS and a stock one and I can't really hear too much of a difference. I think the Op Amps made most of the difference, being a crisper top end.