Fast idle:- Air leak somewhere! Mine will close down to a clunk, clunk, clunk, clunk idle if I let it.

Muffler:- Zimmerman tuned stainless muffler and headers for Zenoah 62 fit (with bolt holes slightly opened at the manifold) and are very quiet and powerful. Pitts won't do much unless it's a big one - your limit is quite high so it should achieve that tho. The standard muffler is many dBs too high - even if you live at Cape Canavaral !

20" prop is too small. Even with a big pitch. Squeeze an inch more clearance from somewhere if you can.