How about an RCV engine? If you're not familiar with them, the prop shaft is on the end of the cylinder sleeve, which rotates around the piston. The intake, glow plug, and exhaust line up in sequence with a port that also is part of the cylinder sleeve. The cylinder is actually pointed toward the front of the plane, so there isn't anything to stick out except the exhaust. The other thing about this engine is that the prop turns at half speed to the crankshaft, so you can use a larger, higher pitch prop. Look at www.rcvengines.com