clarky,
1. The first thing that I would do is remove the muffler and check that the port id not blocked up with carbon.
2.If blocked up clean the carbon out making sure that the piston is covering the port so that none of the carbon goes into the cylinder area.
3. If not blocked check to to see if the piston has any scoring on it. If it has you have a bit of work ahead of you.
4. If no scoring try to start the mower and if it starts easy then replace the muffler.
Try these things first before we go any further.
Regards,
Bruce