Try to avoid removing the muffler from the head
Muffler bolts are prone to breaking which turns a 2 hour job into an all day nightmare.
Pull the carb off at the manifold, replace the O ring but don't touch any of the linkages , just lay it aside .
If there are signs of the leaking gasses eroding a channel in either the head or cylinder then lap them with some wet & dry on a sheet of glass or a concrete finishers /plasteres float \
The surface is not flat as they are cut with a flycutter rough in a mill deliberately to grip the gasket .