If you are swapping the entire front end, I'd be 98.64% confident that the '12 450 front end would physically bolt right up. I've put an '07 450 and now an '11 450 front suspension on my '04 kx 125 framed bike and the stem lengths are the same.
The bigger issue for you could be the length of the actual forks. The 450 forks were longer than the 125 forks. I had to run the 450 forks so high in the clamps to get the correct front end height that I ran out of clamping area on the lower clamp (if that makes sense). It wasn't that difficult or expensive, but I shortened the 450 forks to get the proper fit.
I believe the '11 and '12 450 forks are identical. I've revalved the '11 forks for woods riding, but they are the best I've ridden.
