I think a voltage reg is a good start, but it is easy enough to put an oscilloscope on the 12/24V bus and see if it is clean, and you can
buy a cheap scope for the price of a voltage reg
But you need to eliminate any magneto issue, because of the capacitors (a.k.a. "condensers") are gone then the contacts will wear quickly and in an extreme case you could get an engine stoppage. Don't ever mess with mags...
Sallyann1234's suggestions are good too. Avionics wiring is mostly done by people who have only the barest understanding of electronics (they are nearly all just wiremen) and a lot of it is bodged.