Originally Posted by
martymayes
The "leader" by engine brand is Volkswagen flat four air cooled engines by a large factor. The limiting factor is horsepower, getting reliable power above ~50 hp is questionable. Respecting the limits, it's probably the best bang for the buck.
There are some other conversions that look interesting. Would be nice to see some data-Conversion cost, operating numbers and reliability but the latter is somewhat elusive. It simply takes a long time for a hobbyist to rack up 3-400 operating hrs.