Besides the obvious J arm differences, the only mechanical differences in parts are the swingarm, carrier, sprocket hub(which must all stay together) and the front brake calipers, hoses, rotors, front hubs, and spindles(which also must stay together). By stay together, I mean that you can't interchange them with later style parts. So, in other words, you can swap all your 87 parts(except J arms) to an A arm frame and everything will fit. Just don't try mixing the above mentioned parts with later parts. Also worth mentioning, the '90 was a J arm bike with late style front brakes, spindles, etc. There are several electrical differences over the years, but that's a different issue.