I have an 87 aside from the j arms there is less gussets around the subframe, frame connection area. My 90 has them though, and where new shees mount the foot guards I dont have these mounts on either 2.
The rear sprocket hub has four arms instead of six. the front brake calipers are larger. The spindals are different for caliper mounting. There is no brake switch at the pedal.
The electrical harness is different plug and wire wise, and this rev limiter I hear about dont exist, I dont think. Well Its never cut out on me in any gear, maybe in neutral. and the gearing and tire like he said. thats all I noticed.
edit
acc to clymers 87-89 = 401
90 = 408 must be gusset and smaller calip /spindals
91 += 410
add some saller hubs newer style calips/spindals small tires get rid of
coolant bottle. Cut off the mounts w/ rubber dampner for the stock exhaust and
the torrs box mount. Id guessin 30 lbs lighter more or less.