ORIGINAL: dasintex
Not sure about 20 size gas; but on a 50-60 size gasser, a guy had a flex mount of some sort, and the vibration that was created broke the muffler, it was explained that the engine should be mounted solidly onto the airframe, and that the wings of the plane act as a vibration dampener, in short the harmonic vibration that the engine creates is transmitted and dispersed through the wings, to test this, try running an engine mounted solidly on fuselage without the wing on and it will vibrate and bounce around wildly; on the smaller size gasser it may not be a problem, if it does change the mount out for some metal mounts.
Most "flex" or "soft" mounts are junk. They simply transmit too much energy into the airframe and really don't do much isolation.
In my experience the only such mount that makes sense is a Hyde mount. I make my own mounts in Hyde's style for either beam mounts or stand offs. I've soft mounted a DLE55 this way and it truly works wonders. Of course every other smaller gassie is also soft mounted in my planes. BUT I don't use mufflers in mine...only headers and tuned pipes, also soft mounted. And BTW- these soft, rubber isolation mounts are not that heavy-for 30cc class, the mounts weigh around 2 1/2 ounces. The one for the 55 weighs 4 ounces.
I have various threads in RCU that discuss this. Even have one that teaches you to make your own. I abhor hard mounted engines unless they are smaller than 40s