The last semi-successful car-to-aircraft engine conversion (just my opinion of course) is the Thielert / Continental range of diesel engines, based on Mercedes automotive engines. Although they are nice to fly behind, the engine is a lot more complex than a comparable Avgas-burning four-cylinder and there is hardly anything of the automotive version left in the final product. They also need to use a gearbox, and this component is what you'll be opening up regularly as there are several parts that need to be replaced, and replaced again, before the engine itself reaches its 2100 hour life limit.