You could try a "secondary" muffler and use a piece of silicone tubing to connect to the header pipe. Just Engines sells several of these. Or you could get the Top Flite in-cowl muffler meant for their scale planes and do the same thing.

4 strokes, in general, are not finnicky about muffler volume and back pressure the way 2strokes are. You'd be okay with just about anything plumbed onto the exhaust header.