When I have trouble finding muffler screws I retap with 4x40 or the next practical size. I use RTL Fasteners allen head screws and have no problems after that. As to the long muffler screws I remove the nuts and put a good lock washer and some loctite on them. Haven't lost one yet. I also drill very small holes in the flange that attaches to the engine and wrap a wire through them and back around the cylinder. Never lost a muffler or part of one yet.