There is a fellow in the UK that makes piston rings for model engines. So we now have a new source for piston rings. We can get bearings from several sources so that should not be a problem. Other than that you might need a parts engine if you need other parts such as a piston, cylinder, etc. But one can make a connecting rod if need be. So I don't see a problem with keeping the engines and still using them. Usually with good care a engine outlasts a few airplanes anyway. So it isn't a problem then.

Basically, if man made the thing in question, then man can make parts for it.