RE: muffler gasket?
If the mating surfaces are clean and true you dont need a gasket.
If not they can be faced on emory paper and glass.
If you're not inclined to do that a very VERY thin smear of high temp RTV, but betware, if you use too much and it squeezes into the exhaust path, it can flake off, get sucked into the chamber during the intake stroke, and will instantly snuff the glow plug.