Undo the engine and rotate it 90 deg so the plug is pointing backwards as it is supposed to be fitted.
If this has the pull start in an awkward position then undo it and rotate it as well.
Pulling directly backwards wors a lot better than pulling to one side.
having done that you can fit a standard tecumseh muffler and forget about the custom exhaust.
All of the aftermarket parts suppliers do standard mufflers for your engine so that will not be a problem.