Here is theader and muffler I made for my kart. I still need to weld on the flange. The muffler has some fibreglass inside it. I will see if it is too restrictive, but it certainly will do a good job of muffling!
Stage 1 i/d 22mm ~7/8"
Stage 2 i/d 25mm ~1"
Muffler i/d 22mm ~7/8" o/d 50mm ~2"
At the end of stage 1 there are some holes drilled. I heard this is meant to break up the airflow a little and muffle the sound slightly. I need to make a muffler clamp
The muffler was made from a section of 50mm o/d pipe, a section of 22mm i/d pipe with a load of holes drilled in it and a stop in the middle, and two 'end caps' made on a lathe. It is held together with tack welds for now.
Here it is-