Pretty much every express or QT50 that comes through I make a pocket bike pipe for.
The metal is HELLA thin, so welding it can be difficult. The up side to that is it ends up being a really light weight pipe so the brackets you inevitably have to fab up don't have to be heavy duty.
The sound they make is pretty unique too. Its like a mix between a super load popcorn maker and BBs being shot against a dumpster.
You can get them for like $30 shipped. Mad cheap. I think one is going to be going on my dioree build.