Get it... Bolt on and go ride. Power is similar on all engines. There are diferences, but in my opinion, Is nothing to worry about. 85-86 engines are identical excluding the carb. 87-89 motors are long rod motors (4mm) and have improved port design and some tranny gearing change. I have them all and my 85 (short rod) trail rider have the best power over all and rideability with no ill effects. It's all stock but the exhaust and reeds.
There are some diferences in the later pistons as well.
I may miss a lot of things, but is it really that important? I know some of the guys will chime in soon.
Man, just go get it. And enjoy.
TORKER.
THOSE WHO SAY IT CANNOT BE DONE, SHOULD NOT GET IN THE WAY OF THOSE BUSY DOING IT...