Thurman - did you attempt to re-assemble your old muffler and try to crimp it together? Or is it to far gone to salvage? As far as new ones, I think CC Specialties has the 10-14hp quiet version, which is similar to the 16hp version but I believe a little smaller and I believe has a smaller inlet with a 45 degree angle vs the 90degree (straight) inlet on the original. I also believe the inlet diameter is about a 1/4" smaller but not sure. So it would take some adapting. please let us know what you figure out.
Also, backfire on a 16hp is not good in my view. I don't recall how long you've had this unit now but it could have been carbon build up in the muffler itself that tends to ignite, but that backfire is usually right after shutting off a hot engine. If your backfire occured at start-up then it could be carbon build up on the head and top of the piston - you have to remove and decarbon the head/piston which Kohler recommends every 500 hours anyway. It could also be a carb adjustment, or the timing. If it does it again I'd be concerned and investigate the issue.