Well, if you think about it...
The piston kit part number is the same for both bikes? So then it follows that the piston in both those top end kits will be the same. Piston kit being the same, the rings, wrist pin, and circlip will also be the wame. The remainder of the "top end kit" is comprised of the top end gasket kit and the wrist pin bearing. If you have a 90 top end, then logically you want to get the 90 top end gasket kit, should there be a difference between those gaskets and the 89 gaskets. If there is no difference, you still get the right stuff.
Right? That would be my best assumption going off only the information you provided, and not researching it for ya. ;)