Air box, battery case and tool box will fit all years.
If you need the front forks, you can use any year, you just lose the air adjustment on the newer models. You can also swap out the Master cylinder for the newer round one which will also make it easier to get a brake lever (the two go together and the rectangular Master cylinder requires a specific lever)
Wiring harness is a bit trickier though. You can go newer, but you can't go older. If you go newer, you lose your turn signal cancelling, if you go older you have a more serious problem of having an open circuit.
Here's a write-up from the yahoo group about that
A quick update for any of you who might try to outsmart Kawasaki. The
wiring harnesses from 1985-1988 VN700/750 models are *not*
interchangeable with 1989 - Up VN750 models. The 1985-1988 bikes
have both a Reserve Lighting Unit and a Turn Signal Control unit.
Beginning with the 1989 model year Kawasaki simplified the wiring
with the elimination of the Turn Signal Control unit. Because of
this there is no way to overcome the routing of the wires used for
this unit leaving the signal circuit open. I hope this saves someone
else some time!
Patriot Guard Rider
2000 VN750 Sere (Serendipity)
1990 GL1500 (Ole Blue)
1986 VN750 EVie (project bike, heavy custom)
VROC # 11628 / 25000-H
ARVROC # 12 Coordinator and Crowd Control
OKVROC # 18 (H)
TNVROC # 45 (H)