I did my gen-3 frame the same way my cousin "hoofarted" did his gen-2 if you look in the builds section and it came out real nice. othere than that pretty much everything else bolts right up, no issues with the tank, the rads don't really require any modification except bending the crossovers forward and getting a 3/4-5/8 adaptor for the lower hose. I haven't done the airbox on mine yet, but it ooks like swapping in a gen-2 boot lines up well and the airfilter and bolt pattern are the same, so it looks OE. exhaust mounting will probably be the worst part, but you could just buy a gen-3 specific exhaust from TTM Mike and not have to mod anything
honestly, I think the gen-3 is the easiest and cleanest conversion. the gen-4 is a major PITA, the gen-1 although easy as far as frame mods, the swingarm spacers don't come out as clean and the tank requires major reshaping and a shorty plug is required.
plus, the gen-2 and 3 have a similar rad. setup as a 500, so the stock upper hoses modded work, the gen-1 and 4 either require adding an upper inlet on the right are a Y with more hose mods.
all in all, I think the gen-3 is a winner! an '02 is almost the exact same bike as a '07, so body work, suspension everything is great and availability on parts and accessories is bar none.