I'm sorry but I politely disagree [8D] : there's always a way, but it won't be the easiest one...
If someone wants to quieten his plane, he can do it, preferably at building stage (it's easier), and it works for engines ranging 0,8ccm to 200ccm, 2 or 4 strokes or gas - it's all a matter of time and effort. What one needs is a soft mount designed for the engine displacement, one or more silenced muffler(s) and a couple of props to experiment with. I have seen larger gas engines (80 ccm) run almost like electric in a 3D aerobat, barely noticeable at idle and with a pleasant sound in the air, power not lacking at all.

Regards
-Fabrizio