As you will be changing only the fairing and fairing frames, then of course you'll not be changing the wheel so the wheel will be the same (but for reference the 900 uses a 16" front and 18" rear from 1984-1989 and a 17" front and 18" rear from 1990-2003).
The GTR clocks will not fit a 900r fairing frame as they use different mounting points and the GTR clocks are much bigger! The wiring and plugs are the same for both sets of clocks, so a set of 900 clocks will plug straight in. (Man of Blues is right about the side stand light though, so a suitable mod will have to be made).
Once you've done the conversion, it's really a case of how much you do as to whether it will be easy to swap back and forth. If you go the route of only swapping over the top fairing and leaving it semi naked then other than making the cutout for the rad top hose, it's straightforward and pretty easy so summer/ winter changes will be a cinque. If you go the full fairing route and change both fairing frame and rad/rad frame then it will still not be a major headache but will involve swapping rads etc.
Just for info purposes the GPZ900R was sold in the UK from 1984-1996 so there are plenty more examples there than over the pond, spares abound on fleabay, but plastics are getting hard to come by, so a good alternative is a fibreglass replacement (very few people make replacement fairings for the 900 now) one company that makes replacement fibreglass fairings for a lot of older machines is skidmarx in the UK. prices are erm reasonable. There are also numerous specialist parts and tuning manufacturers in Japan, basically they have it all.
