I own a 1975 Norton Commando, Mk111 Roadster. Had it for 20 years now, needed a lot of love when I got it. Then a complete rebuild after a couple of years, gudgeon circlip somehow de-materialised and allowed a gudgeon pin to wear a track in one bore. A strip revealed the hardening had gone off the cam, as well, that's all she wrote. As you said, they need to be easy to rebuild, because they will need it more often. Say what you like about modern cars, and planned obsolescence, and difficulty of working on, but I'll back the cheapest Hyundai motor against the best that Holden or Ford built in their earlier years for longevity. Basic servicing will see them go at least 200k's without problem, more likely twice that, a grey Holden was burning oil and worn out before then. Difficulty of working on and more complexity is the price we pay for ever more horsepower, with better fuel consumption, and cleaner emissions.