it depends on the type of fuel tap...
If it has two screws on the front of it, on a small plate behind the tap itself, then yes, you should be able to undo them, replace the O-ring behind the plate (usually where they leak) and you'll be good to go again.
If however, it has either punched pins, or no plate, screws or pins whatsoever, then you would have to replace it, in which case i would suggest you take it out of the tank, and into a bike shop, as there are several different types of fuel taps available...
I'm pretty sure the Mini Deserts run the same tap as the old thumpstars, in which case you will have the screw type, and will be able to put a new oring in ...