all the ssie have 25mm front axle. the ssie started in 1998 in most of the world and possibly 99 in the usa.
carby ss have 17mm front axle until 93 sometime, then 20mm. 94 888 also have 20mm front axle.
the problem with using a later wheel is that the bearings are all different widths, so the wheels are machined differently to take those widths. if you tried using a 25mm axle wheel with 20mm bearings the 20mm bearings are 2mm wider and so the axle won't go in as far and the speedo drive will offset the wheel.