Hi. Just in case you didn't solve your problem, yet, I've been using with very good results the modern 40-45-51 Supertigre Muffler on my Webra 50. With a minor holes enlargment (even if I hate to enlarge the holes!) the muffler adapter can be bolted to the Webra, and then you also have the possibility to rotate the muffler at your liking.
Give it a try, there are a lot of Supertigre mufflers around!
Beppe