The diesel aviation engines have a gearbox, and on the DA42 (the only diesel I have flown) they have a light composite prop and the engines are mounted on movable mounts. If you grab the spinner on the ground, you can wiggle the prop left and right and up and down. It is a bit disconcerting the first time you go into turbulence as the prop "wobbles" but it is very smooth in the aeroplane.
The engines themselves are variations on automotive engines, as the dipstick in the DA42 is the same as the one in my Audi
Engine mangement can be handled by FADEC which does a much better job of it than a human (as long as it doesn't stop doing that job!). Perhaps if someone came out with a relatively high power petrol engine/FADEC combination, it would be lighter and more economical than a Lycoming and less prone to damage from things like shock cooling / mishandling?
Actually that describes the Honda/TCM project exactly, I wonder if they are continuing with the project?