I know the bikes bodywork changed in 2001 but I think I recall reading somewhere that gas tanks from 2001- 2003 would fit the older bikes.  Is this true?  Will any of the other parts from a 2002 fit a 1991? like body panels, seat or engine clutch cover?

Yes it will fit.The only thing is the front of the tail plastics don't have anything to attach to.I made little brackets and put the rubber gromets in.It mounts with the tank on the frame.When you change tanks you'll understand what I'm talking about.

The motors from 89-02 (no 03 in the US) are all the same, except color.

Getting the 01+ seat and tail plastics to fit require fabrication of brackets. The mounts are in different places and size. I did this on my 97 using 04/05 tank & tail, using the mounts from a 05 frame.