With the Fiberglass Specialties 4 star cowl you have pictured you don't need to move the engine at all. As for moving the engine down on the firewall for a radial cowl... Doing so should cause no problem aside from losing prop clearance. Just be sure to keep the fuel tank at the right height in relation to the carb... But since you plan on going with a gasoline engine this is not so critical since the carb has a built in pump.