You could drill and tap a hole for the nipple. It goes into the tapered part between the pipe and the wides part of the muffler. I'm not sure what the thread size is, and It may not be important other than to get a hole that the nipple threads will bite into. I don't think this is thick wall aluminum at this point. If you decide to relace the muffler, and could find an O.S 52/61 muffler, it would be my first choice, they have a baffle in them and give slightly more tank pressure.

Don