dirty prop, you fall for the same false comparison that has tripped up so many other folks; Automotive and aviation engine operating environments have very little in common.
When did I make that comparison?
I used the Rotax 912 example, which was developed specifically for small aircraft application only according to the manufacturer's brochure.
As far as I know they didn't take a car or snowmobile engine and adapted it, they started from scratch.
I'm fiddling with a common rail fuel injected Rotax right now and I continually have to ask myself if the complexity of its care and feeding (complexity of the fuel and electrical systems that I have to install) is going to be worth the time and money involved.
According to the huge sales number, looks like the market decided it is.
You're obviously free to use other engines, but with a market share of around 80% the 912 is the de-facto standard for ultralights.