I said nothing about anything other than the quoted power figures. The 21x12 would be a good choice. I guess it would turn 6100-6200 RPM. That would allow you to throttle back to get a reasonable cruise speed and burn less fuel. Less pitch would be slow and you might find yourself flying at high throttle to keep it moving. That AMR Trainer 50 is sure to be a floater, but it depends if you like to fly slowly or not. Unfortunately, trying several props if you don't already have them is an expensive proposition.

Besides, what does it matter what the head looks like when it's buried under a cowl? I'm waiting on a Graupner Taxi 2400. I hope everything fits under the cowl. The engine has to be built before I worry about that much. Diesel or not, it's going to be four stroke.