Ultralights:
I would imagine an aircraft engine would be on the lower end of that scale, magneto ignition, most are carburettors, or rudimentary fuel injection, no quad valve engines, no constant mixture and timing adjustments via real time computer management systems... etc etc.
The Rotax 912 iS (electronic fuel injection) does all this ....to replicate exactly what can be achieved by running lean of peak in a conventional Lycoming or Continental fitted with an engine analyser and GAMI precision injectors.
Overhead cams/four valves/adjustable timing, etc. is not warranted on an engine spinning most of its life at 2500 rpm and 75% power. All that stuff does is increase parts count, complexity, weight and probability of failure.
I am currently deep into setting up a 912 iS and the wiring is quite complex. For example, you need seven switches to control the engine plus master switch if you follow the Rotax wiring recommendations. *
* ECU A & B, Pumps 1 & 2, Start Power tie, Start and emergency battery mode.