Sixze and wall thickness of stock exhaust tubing.

I've had good luck with 3/4" EMT, electrical conduit. Actually I made a tool to swedge the conduit so it slides over the stock, or 2 stroke, if that's what you have, exhaust pipe. I TIG weld it but a wire feed or MIG machine will also do the job. Conduit benders are available so you can fabricate the pipe to fit your frame.
Good luck and you'll get other ideas from members. And yes, the stock pipe is about 7/8" O.D.

Tom