I have now made a new muffler. The pictures tell the story I think. I made it from some exhaust tube from the auto muffler shop in town and some scrap metal from the tip. It works good, about the same noise level as the standard one, but a lot better looking and around 80 grams + lighter.
I ran the engine with it and with the new prop. 6000 rpm on the bench. I will do a RPM check with the old muffler to compare, if I get around to it.