I welded up a few badly buggered screws when all I had for a lathe was my little Unimat, and couldn't make a screw. The hardest part for me was not burning away the edge with the MIG arc. I found it best to insert the screw into a small steel flat washer that had a hole a wee bit smaller than the head diameter, and then weld all around and into the center over the damaged slot. Then the washer and excess weld was turned off in my Unimat after rough grinding, and the slot cut with a jewelers saw, and finished with a slim needle file. While welding, I clamped the threads in copper to protect them, and coated them with some anti-scale compound. Jobs like that became easier when I learned to TIG weld.
I've read about Kutter's method of removing the damaged screw top, and silver soldering a piece on to create a new head and slot. It sounds as if it would be quicker and easier than welding.
For most of the domestic guns I bought that had badly damaged screws, it saved a lot of time and money to just buy a replacement screw from a firearms parts dealer like Numrich, or the parts guys at gun shows. I've laid in a pretty good supply of various size oval head wood screws for guns for things like forends, trigger guards, and buttplates, which were bought very reasonably at gun shows. For common field grade guns, having the screw slot a bit out of time was much better than looking at the buggered screw, and often, it wasn't even necessary to blue it. Peening the damage back into place works well for screws with minor damage, but bluing afterward will always be necessary. The labor cost of having a gunsmith repair or make a new screw just might not make good economic sense for a lot of lower grade guns.