RE: What is a good muffler for the OS .50 SX?
You can get similar results to the Tower Hobbies Muffler by removing the baffel and adding about 2" of 1 1/4" or there abouts thin wall aluminum tubing between the muffler halves. I have done this successfully on several 40 to 55 size engines. You can gain as much as 1000 rpm.
There is also the mousse can mufflers if you want to build one