Thanks for the advice. If this part doesn't belong there, any idea why the exploded views of the exhaust on mopedriders.org and themopedjunkyard.com show at least two rings/washers between the exhaust and block? I thought this might be one of them.
As for McMaster, I already tried looking there, with no luck. They do have some woven ceramic gasketing material that might work. I looked in Grainger and MSC as well. I'm thinking about getting some small diameter (1/8") soft copper tubing and bending it into a hoop, or else annealing some 3/4" ID copper pipe to make it maleable, cutting a 1/2" length of it and hammering the ends so they flare out.
Another question for you: can you get the spark plug out without removing your engine from its mounts? There's no way I can get a long socket between the plug and frame. Is there a commonly available tool for doing this, maybe a wrench with a zig-zag handle?
Thanks for your help!