The threads are M5x.8 and any bike shop will have a tap that size. It's possible the threads are just cloged with paint and a tap will clean them up completely.
At worst, if the threads are hopeless, drill the hole out to just clear the bolt and use a longer bolt and a nut. If you have clearance problems on the inside of the fork dropout, insert the bolt from the inside and use the nut on the outside. BTW, the nearest SAE size to M5x.8 is a #10-32, not a #8 bolt.