You'll have to bore them out to a bigger size then find a pedal with a thicker thread that will fit in it (this will only work if you're running 1/2" pedals and go up to 9/16").
Another option would be to helicoil it, but I'm not sure how easy it would be to get your hands on a 9/16"-20 helicoil kit. Maybe a bike shop would have that size.
Or, you could weld more material onto the crank hole, drill it out, and retap it to fit a pedal.