This is a pretty good discussion of aircraft powerplant considerations for today’s light planes.
Only thing Id add to it is basically economy of scale. Ie if aero engine manufacturers had the economy of scale of the big car brands, then the price of aero engines would drop.
You can get a second hand but practically new sports bike engine for around $2000. We are talking Suzuki Hayabusa 200 horse power (This motorbike does 330KMH). I think this is an ideal candidate for an aero engine on a budget, but it needs some redundancy features implemented some how and also a way to adjust mixture with altitude.
You can try then using a car or motor bike engine in a plane. You get cheap hi-tech engines. But they are lacking the features that are usually found in aero engines, usually centred around redundancy and reliability. Also car engines dont really focus on mixture adjustment for regular trips to higher altitudes. Traditionally aero engines use two magnetos for safety and redundancy. So you get your spark for your spark plugs from magnetos, so a battery is not required, but in a car engine, the spark comes from the battery and the alternator charges the battery. So if the single alternator goes, or if the battery fails, then the spark stops and so too the engine.
Here is a replica Bugatti powered by two Hayabusa engines!