There seems to be certain areas on an aircraft engine and systems which cause problems. If it were possible to remedy some of these we would have better reliability and safety. Here are some of the things I've come across:
Camshaft wear (mostly Lycos).
Valve guide tolerances.
Exhaust port erosion.
Vacuum pump failure.
Cylinder Head cracking between inlet and exhaust valve seats.
Oil pump failure.
Pushrod tube sealing.
Crankcase cracking (Continental).
Crankshaft rust/pitting.
Of course, a lot of these things are down to lack of use but perhaps that should be factored in as well.