Frames: All the same but 88+ have extra gussetting in the rear for more strength/durability
A-arms: 87 are square on top, 1 mounting point (1 long bolt), 88+ are round, 2 mounting bolts. Lowers are same for all years...
Spindles: 87 and 88+ are NOT the same... different length on top
Motor: 87 has 6 studs and larger 5 bolt intake, 88 has 7 studs and smaller 6-bolt intake
Swingers: All the same
Suspension: all the same
Steering stems: 87 are different from 88+

I would look for an 88+ if possible... but just MY OPINION. There are A LOT of 87's out there, so you may be able to find a better deal on them though. Join quadracehq.com .... everything you need to know is on there. There are a lot of power/reliablilty upgrades that can be done to make these things solid bikes.
Good luck w/ your search.....