I have made a few mufflers for small engines just for fun. I have one on
my 125 Honda that is so quiet you can hear the valves operating. It is a
tube within a tube.
Go to a camping supply place and buy a camp fuel bottle. Very nice spun
aluminum with threaded neck I believe those small mufflers have a pipe
thread so find a piece that fits the motor and the neck of the fuel
bottle. Drill about 30 holes in the pipe and slip it into the bottle.
Drill one small hole on bottle near the end about half the size of
exhaust port.
You need to make a plug that fits inside the pipe so you can attach it
to the wide end of the bottle with a small screw.
That's it. If seems like too much back pressure drill a few more smal
holes.
Total cost about 6 bucks