agreed, either Yamaha on 24 spline or Suzuki on 26 spline. Most 87 models did have 24 spline axles, and some 88 models did as well. I speak from experience, I had an 88, bought Z400 hubs without even counting the axle splines, found out the hard way my 88 had 24 spline axles.
if they are 24 spline, you can also use Honda TRX300EX or TRX400EX rear hubs. They give you an extra inch width on each side, just like buying a set of G-Force rear hubs. They are 24 spline, but they are just a little different. My axle was a little rusty on the ends, I sanded the ends, cleaned with WD-40 then applied some grease. I had to "convince" one of the Honda hubs on the axle with a rubber mallet and had to "persuade" the other on with a ball-peen hammer. You should be able to slide a Yamaha hub on by hand.
4X115 rear hubs off a Yamaha is somewhat common, but 4X110 from a Honda is the most common because other manufacturers use it as well. Then you can alsways use 9" rear wheels, which provides the biggest selection in tires. Most common is of course 20", Artrax makes some decent trail and MX tires, and the price is unbeatable.