I have had various cases of broken screw, bolts, etc over the years (not on planes) and in almost every case, I ended up drilling the screw or bolt out and tapping new threads.

You can try to cut a slot like the Vision said. Before you do, get some liquid wrench and drip some onto the screw and let it set. If you tap on the muffler, that may help the liquid wrench make it's way lower. Also, as you grind the slot, that will help it to migrate lower.

EZ outs as Aptar described, are also a good idea and work good but usually on bigger stuff. On smaller stuff, by the time you drill a hole for the EZ out, there usually isn't enough meat for it to bite into.

They are both worth trying, but factor in the cost of a new muffler and you may, unfortunately, find out the new muffler was the best solution.

Good luck!