Re: muffler gasket blowout
The welded flange on the exhaust/muffler is almost without exception warped from the welding process when the factore joins the pipe to the flange. You need to 'lap' that gasket surface to get a good gasket seal.
Go to an auto parts store and ask for gasket material that will take exhaust heat then cut your own gasket. Torque the nuts to about 60 inch pounds then check them again after the engine has reached operating temperature a couple of times.
I've found that gaskets made from 1/8" thick material will perform very well. Gasket sealer isn't necessary if the gasket mating surfaces are flush.
One trick to get muffler/pedal crank clearance is to enlargen the holes in the flange so the muffler can be rotated slightly to clear the crank.
Don't ride the bike with the muffler detached or leaking badly at the cylinder. That can cause a lean condition and damage the cylinder and piston along with making enemies of your neighbors.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"