Re:
Nope. Leave the steerer well alone.
When you say the diameter of the crown race is wrong, is that at the fork crown? The steerer tube should be tapered there to hold the crown race in place, but the crown race should slide easily down most of the steerer tube until it gets to the tapered section. It will need a bit of persuasion to go over the taper.
Are you sure the crown race is for the same diameter of steerer tube your forks have?
I've seen threads on here about differing headset size standards but I'm no expert and thought this tended to be for the 1 inch size.
Many headsets have split crown races to make fitting easier, do you could cut a slot in the mx crown race, but that may not solve your problem depending on your headset (loose/caged bearings Vs sealed).