I used the stock muffler on the Epsilon and made tunnel (ditch) down the middle of the bottom of the fuse. Kind of a pain but worked ok. I had an OS91FX in a Showtime with the inverted pitts muffler. Engine was inverted. It fit fine in the Showtime cowl. The Showtime is quite a bit heavier so I moved up to a Webra 1.20.