Not a proven option yet, but Raptor Aircraft (a brand new kit aircraft designed and being built by an Aussie in the USA) has a 3.0 litre turbo Diesel which is estimated to put out 350 HP.
Raptor Aircraft Home
These VW/ Audi diesels can be truly disruptive - they can run on Jet A1 and their SFC is half that of your air cooled flat sixes.
Fingers crossed they can deliver even half of their promise.