Yea...youre after an older one thats got miles or has already beend ropped i think. The bodywork isnt that important im guessing...you should be able to find an older gsxr or something that fits that bill...especially if you wait a little while here for season to start back up and the wrecks start rolling in :-p
I'm not sure about the sprockets...I thought you replaced both as a rule of thumb period so the sprockets should come in a pair even when changing the ratios. The nighthawks and twin heads have torque but I still think youre going to have to stick with the standard sportbike base for your goal here...happier for the money spent. Probably going to have to look for a while for a deal that isnt a trashed/junk bike. Those other bikes Im pretty sure are heavier and as you said stretched out more...making it that much harder to pop that wheelie.
To be honest Ive never been much of a stunter as I care about every bike Ive had and dont want to smash it into a pole or drop it in general. Im going based solely on numbers and weight ratios...specs and what most stunters seem to have...