The cheapest glow engine for airplanes that is commonly available in North America is the Super Tigre G-40 ringed engine, usually sold for $49.99 before discounts.

This is a twin needle dual ball bearing design that is reknown for both its power and its reliability. It has a 3-year factory warranty and many pilots fit them with performance exhaust systems and use them for pylon racing.

With such a fantastic engine available for so reasonable a price, it hardly seems worth it to try converting an old truck or buggy engine.