I have swapped the axle from an older rim with small axle and put them into a rim from a modern bike that had a larger axle in order to use an aluminum mtb rim onto older spade forks. The parts dont always swap but some will.
You probably have a fork designed for 5/16" axles, and your new wheel probably has either a 3/8" or 9mm axle. What I'd do is file the axle where it meets the fork's ends; file 2 sides flat, directly across from each other.
Now, you might also mean that you have a fork spaced at 91mm but your new hub is spaced at 100mm, in which case you can either spread the fork to accept it, or see if there are any washers/spacers between the jam-nut and the cone that you can remove to narrow things down a bit....
yep its a 100 MM hub going to have to spread the fork a little going to look for some thiner jam-nuts or sand some down, the rear axle/Hub fit fine no problems there.