I have used 3 AX engines and had them all mounted at the 135 degree angle, so the muffler exits the bottom center of the plane; they all start on the first flip (yes, I use a chicken stick with my 120AX!) with no problems. This angle keeps the fuel from completely soaking the glow plug during ignition, which some say is a cause of engines not starting when inverted. The instruction manual for the 120AX comes with a template to mount the engine at this angle, and it works and looks good.

If you WANT to install right side up, you can do whatever you want to do; it would require just as much cutting, and possibly be even more complicated. Let me point out that the muffler exiting the bottom (and with the use of an exhaust deflector) the plane requires VERY LITTLE clean up at the end of the day.