how about modifying a 100mm hub axle? you could cut the axle and change the spacers to make the OLD 90mm. that would be assuming that the hub/axle you have has the appropriate amount of spacers to allow it. some hubs are wider from flange to flange and use no spacers, those type would probably not work.

for all of the hassle involved it might just be easier to get a new fork. based on the 90mm spacing i'm going to assume it's for a 27" wheel and it's nothing special. this may fit the bill, it's spaced for a 100mm hub, and it's cheap: http://www.amazon.com/Sunlite-Thread.../dp/B0014C639K.
amazon doesn't have it, but your LBS can probably get on through QBP or something.