That happens alot especially in ready to runs, solution get two 4 /40 screws and two 4/40 nylon locnuts they make big size 4/40 and small 4/40 lock nuts ,you will want to purchase the tiny locknuts . put the screws through the muffler ,if you cut new threads its ok because your going to put the little tiny locknuts on end of screw & tighten .
Problem solved. Ps you might have to cut screws with dremel so screw dosent stick out so much , I had to anyway. They never came loose again hope this helps.