It's been awhile since I messed with the B motor, the holes for the motor mounts are positioned the same, as I recall 1 or maybe both of the front mounts are narrower, but could be shimmed to fit. The head is different, the head stay is shorter on the b than the E, but then the head is bigger as well so the upper head mount may work. Bottom line it will go in with minimal mods (shims and maybe different upper head brackets).
Having said all this, you may want to reconsider putting NOS to it, the clutch isn't as strong as the newer bikes, they only use 5 clutch springs, hmm maybe you could get stiffer springs.