Some Caveats. The Miroku made '73 is a very well made rifle. It is NOT a reproduction of a Winchester '73. I does look just like one on the outside and functions just like one, but the gun is not a reproduction nor a "clone" (I hate that term).
I don't play NCOWS. Never have, so I can't speak for there decision making process. I have no idea what drives their unacceptance of the Miroku '73. Except, as I pointed out above, it is NOT a reproduction of the original. It has been re-engineered.
Stroke length. Yes. the throw of the lever could be altered to more closely match the original sample example. But, .... why bother. One method would probably be considered an external modification and dis-allowed (altering the lever). The other method, might also be considered an illegal mod as well (altering the Carrier Block Arm). It may well be, no matter what is done to alter the lever throw, NCOWS might still disallow the rifle on the basis, it's not a copy of the original, nor a close enough copy of the accepted Uberti.
Coffinmaker