the main constraint is the weight. a 1930 design can do 40 hp continuous. surely using the lastest science on it can make do more.
Your going to be surprised... If Lycoming, for instance, could just make more power for the same weight - they would certainly do so. They are paid plenty for their motors. Fact is they are built out of the good stuff and look like the best race engines inside. They build 8.5 litre engines that weigh in the same range as small turbo I4 car motors. While your clearing the trees at the end of the runway, it's nice to know they have more then 1000 hours of testing on a single example. You don't need that, but it's a comforting feeling.
All the car and bike engines with impressive numbers are good for something like 10-30 hours of use on a track. That use includes closed throttle every 20 seconds or so. Very impressive pieces of equipment, but not suitable for airplanes. They also require heavy reduction gears or belts etc. and that's a considerable design effort too. With it's own reliability issues. People do use auto/bike etc. engines in planes, especially ultralights but I think that the success is a little mixed...
Engines as stressed members? That's another subject too. I would worry about fatigue in an aluminum airplane.
I'm not sure turbos make an engine much lighter for a given power. The big help with a plane is that it will compensate for altitude, so lighter in that sense. Displacement doesn't cost much in weight just by itself.
A Lycoming 360 CID weighs about 295 lbs. complete without propeller and makes around 180 HP, I think. The Rotax etc. ones do better, I think, but these all have huge efforts into them.
if only those certified engines can be bought and maintained at much lesser cost. the most common reason is the cost of certification and that the homebuilt market is too small. so the only choice is design a conversion properly in a way that land lubbers get the benefits too. there is a ridiculously big market for aftermarket vw parts. i think an engine that can produce 75 % power continuously would make for good marketing. surely a machined fin porsche style head will sell.
in fact i want to start with proper cylinder head with good flow. lots of fins and really thick exhaust valve stems.
all engines in sport planes are structural. propellers are connected directly. redrive or not. most conversions just put a prop hub on the pulley end. it can take a few hundred pounds of thrust for a long long time before cracking and departure from the engine.
i dont like those methods though. i think a proper reduction drive is needed. no engine can make resonable power at the prop speeds i need.
turbos with ball bearings and water cooling is the state of the art and those are heavy. add to that the lubrication and the wastegate then the blow off valve, it just wont do.