I decided to give this thing a try. First I want to attach some npt threads to it so I can thread it into my existing pipe.
If it is still too loud, ...this might be a good base to build a custom muffler from. Make it come apart and go back together again, then drill some more holes or wrap it with better material. Who knows, maybe it will sound awesome without modification?
21mm = 0.83" inlet
2.4" diameter x 11" long
"60mm dia Shark Mouth Exhaust Pipe Silencer Muffler for Motorcycle"
21mm is SUPPOSED to be appropriate for 50-80cc motors but if it turns out to be worthless, I'll try a larger 28mm one (usually for 80-125cc) or a 38mm (used on 125-160cc).