I have never had to deal with this problem with a lock, but I have had to when rebuilding farming machinery. My solution was to silver solder in a screw to act as a plug. Then I re-drilled the hole, and re-tapped it. 45% silver solder is extremely tough, and if you get a good joint fill, it shouldn't be a problem. You can then re -drill and re-tap the hole. I then use my lathe to make a new screw, and all is good.
But it doesn't take a lathe to do this... A lock screw is short and easily fabricated from cold-rolled steel you can get at most hardware stores. You can mount it in a drill, and cut it down with a file as it spins. When you get close, stop and check your work with calipers. This is the easy part.
Now for the hard part... You have to know that your hole is straight and parallel to the face of the lock. Unless you have a well set up drill press, or a mill, this is almost impossible to do. I have had at least one lock that was not drilled at 90x90. My solution in that case, and it was the sear screw, was to trim the toe of the sear, the part that was closest to the plate. I would see if this might correct the problem first, before re-drilling. It is easier to take a slight touch on the grinder, rather than the filling of the old hole and all I said above. As 20th and 21st century people, we tend to try for perfection, and I know I am certainly a victim of this myself. But the original smiths were much less concerned with perfection. They were concerned that they sold guns. Today, we wouldn't sell a gun that wasn't close to perfect, and that is as it should be. I think the old makers would be proud of us for trying to get it perfect. Good luck with it.
Matt
