Almost all front hubs are 100 mm, although I read a post on another site about a guy trying to fit a Chinese-manufactured motorised front wheel and getting into trouble due to another hub width.
Steel however takes to spreading and cold setting nicely, so I'd have no qualms about simply prying the fork apart until it'd accept an normal front hub.
On further thought I think I have had a really old bike that came with plenty of spacers between the locknuts on the front wheel too, so maybe there are some narrower hubs around.
Rear hubs with external gears I believe don't get any narrower than 126 mm. But unless you feel like doing some cold setting there too(sheldon brown has a good instruction) they're far easier to tinker with due to the big stack of spacers usually found on the NDS side of the axle. If it's one solid spacer, replace it with a smaller stack made out of ordinary washers, redish the wheel and ride.
If possible you might want to remove a spacer/washer from the drive side as well.
Track hubs (can) have 120 mm width, (if you want to turn it into a SS/fixie) and internally geared hubs can be even narrower.