Here's how I do it. I mount the engine and then take a stiff piece of card stock and cut out the shape of the cylinder head. Cut the board about 2 to 3 inches wide and long enough so when it's on the engine it's over lapping on the fuselage by a good distance. Put some alignment marks on the fuselage for later. Now remove the engine and put the cowl on, take the board that you made and match to the marks on the fuselage and tape it in place so that it's laying on the cowl. Now you have the location where the hole for the engine should be cut.