I have a DP Ultimate and am using a YS 140 FZ. more than enough motor. I used a hyde soft mount that he custom made 5/8" thick and mounted directly to the stock firewall. As far as a muffler I used just a straight pipe to avoid any weight shaking with the soft mount. It is a little loud but an awesome sound. Very similar to a Harley ideling but really sounds like a true radial engine in flight.