Yeah, like he said...Don't remove, or try to remove that exhaust elbow.....you don't need to!
Unless you have access to an acetylene torch, don't mess with it. I actually scored a cheap 1650 years ago due to this very reason. Previous owner tried removing the exhaust pipe with a hammer and broke the head. Lucky for me, he still had the large chunk broken off and my boss is experienced in welding cast iron.
As you mentioned before, you can get the pipe threaded and go from there putting a muffler on.