As I stated in my earlier message, none of our models use forced cooling, simply because we run them for such short periods that they don't even get a chance to overheat.
The average pull takes 3 seconds. Including starting and hooking the tractor to the sled that would be about 15-20 seconds running time.
Most of our members let their pullers warm up for a short while before pulling, just to get the engines up to operating temperatures.
We even use inboard marine engines without any extra cooling, without problems.

Setting the mixture / compression with the engine on a test stand using a propeller seems like a good solution, but what size proppeller should I use?

Cyril