Unless you plan to use the stock muffler, you'll find that the Mac has a somewhat unique exhaust port configuration that requires a special muffler setup. AFAIK, Abell hobbies is the only source for one right now. They have models that fit both the bolt on and spring type muffler. They're web address is: http://www.abellrc.com/lgear.html. The guy you want to talk to is Clay.

I have a couple of the 32cc Macs and they seem to be good strong motors. Mine will run a 16-8 to 8000 rpm in the blink of an eye, and it's not even well broken in yet! They are expecially good in situations where the Ryobi engine is going to be too long to fit in the cowl space available.

Kiv