Rotax! 4 stroke high efficiency lightweight liquid cooled engines, fitted with appropriate gearboxes to keep propeller tip speeds down. In many ways superior to the elderly lycontinentals that still infest GA.
Rotax Aircraft Engines - HOME
Also look at Jabiru engines from Australia, slow reving air cooled 4-stroke engines.
The best "automotive" conversions that I've personally seen or flown are generally BMW Boxer motorcycle engines. They give a good power:weight, there are modifications copyable to fit Rotax gearboxes, and they are well supported.
Google "BMW Skyranger" and "BMW Huntwing" for two extremes of how this was done successfully.
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