Back in the 30's it was unusual for an engine to last for 200 hrs. of actual run time. With the modern materials used to build them now days, and the ability to buy parts for rebuilding almost any engine, plus factory service, a decent engine should outlast many airplanes. I wouldn't suggest buying an engine on E-bay or from someone who isn't known to you or a friend. Your LHS might have used engines for sale. At least you have someone to go back to.