RE: Diesel Power
Davis Diesel makes head conversion kits to change over nitro engines for Diesel power. I tried it once in an RC truck using an O.S. .12-R (to some people that will date the time). No water or air cooling is really needed as you need the heat. Yes, it was dirty and stinky and you had to be a lot more careful in the handling and transporting of it as it's ether-based.
I didn't have a whole lot of luck with it and myself and one of the guys at the hobby shop both gave it a go. We could get them running but no matter what we did couldn't get them to run over half-throttle.
Give it a shot as I like out-of-the-box stuff myself. I could be well suited for scale boats if you want slow but an engine with a lot of torque.