I have been recently flying with a gutted muffler. Previously I had a 25cc Homelite spinning a 16x8 prop at 8000 rpm using a pitts muffler. Now using a 30cc engine with the same prop I can only achieve 7300 rpm with a gutted stock muffler. My conclusion is that that unless you gut the muffler entirely you will be unlikely to achieve the same rpms as would occur with a suitable pitts muffler.

I will be able to confirm this on the weekend when I will run the 30cc engine with the same pitts muffler as used on the 25cc. The only difference shall be a spacer, which was required to clear the magneto bracket (my first mistake on the 25cc engine). I fully expect the peak rpms to easily be 8000rpm or higher, this will prove that a properly matched pitts muffler will always exceed a modified stock motor.

All the same, good luck with your conversion. Let us know how it goes.