in '01, the monster s4 was introduced with some "significant" differences to previous monster frames. honestly nothing drastic, but i'd imagine there'd be fitment issues swapping motors between model years.
in '02, the rest of the monster line-up followed suit and got (essentially) the same frame as the monster s4. i think the only differences actually are the mounting tabs on the s4's frame for the radiator.
I'm pretty sure all monsters before '02 were built on an 851/888 frame and the s4 and 02+ monsters are now built on st4 frames. I know that one main difference is the rear ride height/shock/swingarm assembly is completely different.
I wouldn't know from the above emails if it would fit or not man you guys are all over the place. The motor mounts in two places those dimensions haven't changed since 1985 thru 2003. The motor fits the frame all frames, there are no 620 frames no 750 frames and no 1000 frames the mounting points are all the same. The engine cases have changed only slightly usually a little heavier here or there, deeper sump or different swing arm support. As to everything else wiring suspension and what not that can be and often is different but is easily changed if someone is doing some kind of swap.