50 to 60 length?
This might work... you could always just make your 50 tube longer . First take note of how the knotches for the frame and tail rotor align. Then....say the boom is 30 inches long and you want it 33". Take it and cut off 6" of the frame end,put the remainder 27" pc aside. Take a second tube and cut off 9" of the frame side. Then roughen 3" of the outside end of this pc.(the end that does not go into the frame)Take the 6" pc roughen it up outside and inside then slice it down the entire length.Take the 27"pc and roughen up about 3" of the outside of the end you cut(frame side),Take the 6" pc and slide it halfway onto the outside of the 27" pc and epoxy it(use a good epoxy like JB WELD), making sure it is glued to end that will be facing the frame and not over the end with the knotch for the tail rotor. Take the 9" pc, slide the roughened end with epoxy into the remainder of the 6" pc untill it hits the 27"pc. (be sure you have the knotches for the frame and the tail rotor aligned or your tail rotor will be crooked!!)
This method will give you a 50 tube any length you want and if it is inside a scale frame you wont see the coupling. For extra stiffness you can cover the tube with shrink tubing.
Also there are industrial suppliers that carry belts that are for certain machinery that use the same type of belt...look into who has what and what are the lengths available...might be possible to find a new belt cheap...even if you stay with the stocker setup.