Bones is right - the starter
clutches on those bikes are *way* different....
What someone has told you is basically half-correct though - the starter *motor* from a later GPZ or possibly even a ZZR1100 ( they had a lot of internal similiaraties) will be a superior replacement - this is an old trick for Z9's and Z1000's - drop in a starter motor from a B2 or ZX1100A1 - they fit the early cases, but spin faster and so will kick the bike over quicker - this being the 'superior' aspect to using a later-model starter motor.
I really should get around to putting a better starter motor in the Black Pearl, now she is regoe'd for the next 3 months of fun in the sun.....