The mc18 & 21 front wheels are the same, I think the difference must be in the spacers i.e. if you have mc 18 forks you need to use mc18 spacers.
I have an mc18 front magtek in my 21 and it was a straight swap, although I have nc35 forks But the point is their was no difference in the fitment of the wheel when I changed from the 21 to 18 front.