"It's been long postulated that car engines are unsuitable for aviation use because of the high continuous power demands compared to motoring."
That may have been the case 40 to 50 years ago but does not appear to be the case today. The two Mercruiser V8's in my boat are 5.7 liter Chev small blocks. According to Mercury they are designed to operate continuously in the power range 3500 RPM to 4000 RPM and are governed at 4500 RPM max by the prop size. My engines have approximately 1,100 hours since new and run perfectly. Not too thirsty, around 40 liters per hour (for two) at 3400 RPM.
Didn't the replica Southern Cross have V8 engines? VW and Subaru engines have been converted for aviation use. I suspect some of today's light weight aluminum engines and particularly the light weight turbo diesels could be re purposed for aviation use. I wonder what happened to the
Porsche PFM 3200 aircraft engine and the
Chev Turbo-Air 6 engine?