Jeron did a lightened DD G-10 that filled the requirements. Never finalized.The costs of producing a reliable small engine is enormous, I recon that the “Franken” parts should be confined to ancillary parts added to a reliable bullet proof ‘core’ to make it more suitable for aviation purposes.
I know from my experience working with the Suzuki G10 that the basic engine weighs about the same as a Rotax 582, and I’m sure that it is not the only 3 cyl auto engine that is that light. Naturally it would only produce about 40hp direct drive, but add a small turbo and you would have 55-60hp ( the turbo conversion weight is reduced by the fact that you don’t need a muffler). The Suzuki runs reliably tilted over at 70deg exhaust down and can be tilted at 90deg if a large enough oil return pipe is fitted between the valve cover and the sump. The crank being 120deg they are easy to hand start. All 3 cyl engines exhibit an odd ‘wobble’ at idle but are absolutely vibration free at revs.
Rover. Can use big valves. Was reported to be a good running engine.