I use the Slimline sport scale muffler on mine:

http://www2.towerhobbies.com/cgi-bin...?&I=LXFH00&P=0

It doesn't 'muffle' as well as the stock muffler, but it does have a nice sound and it fits well inside the cowel. I used silicon tubing for the last couple inches of the output tubes, so that I can get the cowel on over it.