Everything is free, we humans just slap a price tag on things
Air was once free, now we have the FAA. See where I am going? Sorry, just being a little philosophical. There some truly free things out there; freedom to be stupid and arrogant comes to mind.

To the OP, from what I recall on some of my st engines, I was advised to drill all the way through the muffler bolt holes, getting rid of the threads and using nuts. Worked pretty well for me. Pity parts are scarce, I had great times with these engines (though the big heavy mufflers always confused me!)