Re: honda moped expansion chamber?
2 stroke expansion pipes are as much a science as they are an art. Way too many variables with bore and stroke, port sizes, reed valves or piston port to just whip one up and have any expected results with any given engine. If you picked up any 50 cc dirt bike pipe it will probably hit a bit higher in the revs that that engine would like but playing with lead in length can make a big difference to make it work on your engine. If you can find a book called "2 stroke tuners handbook" from around the 70's it's awesome to get your head wrapped around 2 strokeyness.
If you just want to experiment about any old expansion pipe can work if you cut it into sections and make it slip together so you can play around with the lengths throughout the pipe. Hose clamps work well enough to hold it together for testing then weld it when your satisfied.
Best of luck these old Hondas where a hoot.