RE: OS .50FSR
50 FSR is a sweet engine. I have been told many times that it has better carb then the 40 FSR. The muffler for it is the "same" as your FSR 60 "744" but the mounting holes are dilled closer together. I have both versions of the muffler.
I have a NIB one, and would buy one if given a good price, but it is still a sought after engine so the price is a little high. I like to get the older engines and use them for sport or scale flying.
In hindsight I think the reason the FSR series was so popular was it was the first series be introduced and designed with the muffler in mind and snural porting (not the 10?). They, other then the 10 seem to be long life, with good power and carb. Oh, I did not mean to bad mouth the 10 FSR, I have one that is well worn (repaired many times) and a very nice man gave me a NIB one to replace it. Also, I do not have many NIB engine just 4 or so not meaning to brag above.