Terbo,
Not a pusher. The engine is a chainsaw, which runs opposite of a weedy, allowing me to use a normal prop in a pusher configuration. bigger prop selection. It (the engine) was made in 1986.

Cajun,
The only way I can think of to be able to put a muffler on is to move prop away from the engine, which would require machining of a shaft. I think that would be out of the budget. The local machine shop starts a $65 an hour, not including materials. [&o]

18 x 8 ??? Faster top speed ?? Hmmm. but wouldn't that sacrifice low end torque ( for land running)?

Correct me if I'm wrong: Lower pitch prop depends on higher RPM for more speed? This engine has a max rated RPM of 8000.
At only 8000, would a smaller pitch be less efficient?

Also I've tried an APC 18 x 12 , it didn't perform as well as the MAS 18 x 10.(although there was no discernable change in RPM) I think tip width had something to do with that.