You can use the later shift drum in the early 750, but not always vice-versa. The pins on the shift forks got longer in the K4, while the grooves in the shift drum got deeper in the late K2. The early drum's shift grooves can sometimes then drag on the longer pins' edges in the later forks, requiring some filing/grinding to shorten them a little bit. It's do-able in a pinch.
But, using the later shift drum in the early engines always fits. It has one distinct characteristic that made me do it in my early K2: the Neutral 'bump' is wider and makes it easier to find Neutral, although if you're going racing it also slows the 1-2 shift a little bit as it makes the lever 'feel' heavier if trying to shift it quickly. I swapped mine at 121k miles because my Neutral Bump was getting worn and this would let it slip past Neutral easily when going from 2-to-N while rolling to a stop. For me it was an overall 'win'.
