HI,
I'd live to see somebody make a custom exhaust into a car muffler....yes car muffler....Plenty of capacity (designed for much larger engines) so shouldn't create too much back pressure....May look a little ungainly but should work great....at least on the exhaust side...May have to hide the air filter in a
sound deadening box since I have heard that the intake is loud also....Other things to consider is "rubberizing" the clutch cover and inserting rubber between the cooling fins....
Do all this and you'll definitely be stealthy!
Hope this helps.
Andrew