I'm not sure how similar the PreGen and NewGen forks/wheels are, but the Gen2 500 is nearly identical to the NewGen.
willi777's OP and
my post have a decent bit of detail about swapping the aftermarket NewGen wheels into the Gen2. Like I said, I'm not sure how much that helps with PreGen forks though.
http://www.kawasaki.com/Parts/ can tell you the dimensions of some of the spacers and stuff, which might help at least a little too.