Alexander, you wouldn't need to have the entire exhaust made. Look on ebay, or in the back of a local shop, for a muffler that is close to what you need. Then look for someone who can bend pipe and weld up what you need - a good muffler shop, or a custom car shop that works with hot rods and kit cars can do the work that you need, and they should have the stainless steel tube that you need. They will probably want the entire bike so that they can fit things up.
Explain exactly what you want to the muffler shop, and also give them a print-out, with your requirements. (One of your requirements should be that the pipe fits closed to the frame, but not so close that it contacts the frame. Another would be that it doesn't hit the rear tire when the suspension compresses.) A chain-store muffler shop probably isn't going to want to deal with it, but a small, privately owned shop might enjoy the challenge.
Best wishes. Let me know how the build goes!
Jim Roble
