Sorry, I misunderstood what you said about the coil. I thought you meant the grounding bolt on the coil came loose causing a intermittent no spark.
I have seen the problems your having with your clutch baskets/drums. I have bought aftermarket clutch baskets from comet and never had that problem but did notice that they seem to wear out faster then the original Horstman stuff so I went back to buying there stuff and there’s very little price difference between them. I will say though when I service my clutch I take those bolts out and re-locktite them back in place. If your having a lot of repeat failures one other thing you might want to try is buying a one piece drum and gear assembly. That would eliminate that from ever happening again.
I’ve seen people have the problem you’re talking about with the starter. I think it is inherit to that design. If you remember old fords they had the same problem and used that same style solenoid I remember taking the solenoids apart to sand down the copper disk that would weld itself to the contact point of course now I think there riveted and not screwed together.
I have had the same thing happen to me with used engines. Only raced twice of course what they don’t tell you is they practiced six times for a total of eight hours or the chain broke and it went to 16000 RPM! Unfortunately I find it harder and harder to trust people anymore. (sad) Which is why I switched to buying new stuff its only more expensive if you don’t have a problem with the used stuff, but as you and I have found out its just as expensive to buy someone else’s junk and then have to rebuild it as it is to buy new. Racing is just to hard on things.