not sure how the exhauest flange is attached to the cylinder - if there are studs, you might try putting a jam nut over top the original nut, screwing it down, then tightening one against the other
if the stud isn't long enough, then replace with a stud that is - if bolts are employed, then replace with a stud - www.mcmaster.com
is a good source for anything, including studs -
as to quieter exhaust, instead of fooling with mounting a different exhaust system, you might try doing what i did on my generator with a really really loud 20hp honda engine
as long as you make sure the diameter of the pipe leaving the orig muffler is large enough to not impede flow, all you'll be doing is decreasing sound - and keep in mind, the large the volume of the muffler, the quieter it'll be
the set up in the below pix took me from an ear pounding headache that even my neighbors complained about (and we've got huge yards) to a noise level you can't hear inside the house and standing 10 feet away you can hold a normal volume level conversaion with no need to shout to be heard
for my use, as i had already built the shed, i used a quick detach clamp (really a sanitary clamp from the food industry, but it's 316 stainless and would take the heat) - you can see it hanging off the muffler in the first shot
in the second shot, you can see the stainless flex exhaust line - it just slips into the muffler - it's 2" OD and the muffler ID is 2.5" so it's not even a tight fit but it slides in about 8"