I have pretty much determined that the 00/01 engines don't just bolt into the 02 and later frames. I haven't actually tried it yet(not worth it to me), but another member here said he did and it took some minor machining/fabrication or spacers and brackets. I like the way the 00-02s handle all the same. The 02 forks were a little soft for me. The 00 shock was too soft. I have all of the stuff but I don't think the 02 chassis changes are significant enough to warrant putting a different motor in it.
The 02 is very fast and has just as much "hit" as the 00/01 in my opinion. Probably more than the 00 and very similar to the 01 (I would say more but some might disagree). The hit just happens a bit later, the transition is more abrupt, the low is weaker/mid is stronger or something. I could get over jumps much easier on the 02 but I think I can putz through turns on the 00 better. I really haven't ridden the 02 enough to be an expert on it and I am not the greatest rider as it is. The 02 comes with a 48tooth rear and the earlier ones had a 50. Changing that made them more comparable. The 02 carb seems to be a bit more fussy than the 00, but is more tunable also.
To sum it all up, I think I prefer the riding the 00 and 01 but that is mostly because both of those are scratched up, dirty, and the radiators are kinda crooked already. The 02 is too purty for me to be riding.....I crash too much.
Thanks for the good words Eric!
I'd recommend riding them first if you could, and from what I hear, you need to try and 04/05 RM or YZ. They are supposedly much better.