The engine will need to be run and given some work before you even think of compression testing the cylinders although why bother, if the bores and the rings are good with the bump clearance as near to spec' as you can get things your flying, with the engine hot and working hard the exhaust type is not going to effect piston clash with the head, the time things can start to tap is when the engine comes off load, a straight pipe will let the heat away faster obviously but the din will help nobody, the air cooled engines only need 1/3rd of the blown air of a water cooled engine, to do a proper compression test you need the engine cranking over fast, a check valve is sweet but a gel filled gauge will do fine, I do tend to think you might be making a meal out of I don't know what, the compression target per cylinder per that engine set correctly is printed in a book, read it after the motor is working if you want.