Rotax (Austrian company) actually builds great engines. They are heavily used in ultra-light airplanes with much more “relaxed” certification.
And you are right: in the ultra-light market, we see a giant boom because of lower certification hurdles and actually much safer systems: automated engine control, constant speed propellers, ballistic parachutes, etc. But Maximum Take-Off-Weight (MTOW) is 650 kg (Germany) that leaves you with a skinny wife/husband, some gas and light baggage...
Airplane engines are operating in tougher environments than car engines - hence bigger tolerances and less sophistication in some parts. They also must not fail. I mean, the engine has to be ok with starting up at 100 degrees F on the ground, climbing at maximum power to 10’000 ft, with below freezing temperatures within 15/20 min and descending down again with an urgy pilot pushing down the cylinder heat temperatures from 400 to 200 degrees fahrenheit within minutes (due to power reduction and increased speed cooling the engine).
