I have not run the mods on a 41, but have seen a couple, and they were strong engines. Saw one an American Eagle Spitfire, with 84" w/s, & it pulled it around with authority. You can get the carb & muffler from B&B Specialties: http://www.bennettbuilt.com/eng10.htm
I have seen the new style muffler you mentioned, I believe it would be about the same as the B&B muffler, or close.