I put a 2000 kx500 tank, rad shrouds, seat, and rear fender on my 1990 KDX200. Had to fabricate a subframe and shorten the silencer. Other than that it was straightforward.
Jason
I don't know about the complete seat, but the foam and cover are the same. You can see a pic of my '94 200 wearing KX plastic by clicking the "my gallery" link near the bottom of this post.