The best thing to do right now is spend a couple of dollars and buy a threaded 4-40 rod. Cut this the length you need to run from the front of the muffler to the rear end. Just use this with nuts on the front and rear to hold the muffler halves together. This is an improvement anyway. While your muffler is apart you can take out the baffle if noise isn't an issue in order to game a bit more power. Look over the mating surfaces of the muffler before reassembling.

If this isn't your problem, come back here to find out what you need to do next.