So I took my lunch break today and made a header. Basically two pieces of tubing from the trash buggy butt-welded. I didn't measure ID but it has to be 1" or greater. A 30mm hole in the mounting plate wasn't large enough for the tubing to slip through.
I didn't realize that the extra muffler I have lying around is stainless and about 1/2" larger than the tubing I'm using. I did a quick run with no muffler. Very little impact on top speed. Looks like it actually hurt top end. Felt like it took longer to wind out, too.
I've got a 125 main in the buggy currently. From what I've read a 125 is often rich even with lots of mods. I can try going richer. I haven't done any chop tests - I was planning to do that once I got all minor details sorted out (header, pipe, etc).