A while back I was flying & the screw & nut loosened up on my OS 46FX & the rear section of the muffler fell off. One of the guys said "just go to the auto parts store & buy a cheap battery post cleaner; they fit fine" ($2 cost average). So I didn't want to spend the bucks on the OS replacement & thought I'd try it. I drilled the small hole in the cap, & a 5/16" dia. hole in the bottom to vent the exhaust. If you push the end onto the stock muffler & add a little nut lock to the joint, it will seal & hold fine, with the proper length of 4x40 all-thread with lock nuts on both ends. I put the nut lock on all fasteners to keep from having to do it again. You may need to file a notch in it to clear the fitting for the fuel hose off the muffler. (Or else put it together, then drill the exhaust hole where you desire)
I found that common "Gumout" carb spray makes the parts easy to dissassemble, if needed. I had to do it on another plane with a cowl where the exhaust came out the bottom.
No problem, just get a small peice of tubing & solder it in the 5/16" hole you drilled. Instant Pitts style muffler. The pics are of a simple make (not the Pitts) I carry an extra predrilled cleaner cap in the tool box all the time. A length of 4X40 threaded rod is good to have around also.