Front spindle was thicker in my '94 one than it's '93 predecessor. I know because I kept the black wheels as I preferred them to the goldish ones on the '94. I do have a another '94 900 which is currently stripped, although I know where both F + R spindles are, it may be a week or two before I see them as I had a new garage door fitted yesterday so everthing is buried and in disarray.
According to Stein Dinse the front axle was 17mm till ‘94 (751633268 bearings) and 20mm from ‘94 model year onwards (400020005 bearings).
When I got nice wheels, they only did the 20mm spindle. Its easy enough to convert to the later larger front size. I have a 1993.