Are the letters are slightly recessed in the dial face? If they are, thin some white enamel hobby paint with some mineral spirits until it's got a watery consistency. Load up a small hobby brush with the thinned paint, touch it to the recess, and the thinned paint will fill up the recess and re-establish the number. Sometimes you have to apply several layers in this process before the letter is completely filled in.
If the letters aren't recessed into the dial face and are just painted on, you can go to a hobby shop and look for some white dry transfer letters that match the font (or are as close as possible) to the ones on the dial face. You just have to position the letter where you want it, rub the letter with something smooth like a dowel rod, and the letter will transfer to the dial face. Make sure you clean off the rest of the white paint of the letters that have been damaged. If you can't find a matching font, you could always remove the rest of the numbers and replace all of them with dry transfer letters so they all match.