RE: stripped muffler screw
Don't no if others would agree and I am not sure what size the screw is but you need to drill out the screw with a drill smaller than the minor dia of the screw. You then may need to chase the threads with a tap after drilling to fix any threads that may get damaged.
This can be hard to do without a drill press or mill however. If you don't have the equipment I would check with a small machine shop and see if they could help you out.
I use to do this kind of stuff for free in my shop when the occasional John Q. Public wandered in. It only takes a couple of minutes if you have the right equipment.
However the shop is now no more.