When I have to fill a hole, I clamp a piece of brass or alloy under the lock plate, and fill the hole with my MIG welder, moving in small circles around in the hole. Now when I have to drill the new hole overlapping the old one, you won't have the problem of drilling in partly peened material loosening.
A carbon "brush" from an electric motor works very well for this too.
Hole drilled at an angle buy the maker. Messes up everything, sear engagement etc.
Fixed. Drilled and reamed the hole then turned the tumbler to fit.
I have used the thread/plug/pean/redrill on lock plates in the past with good results too.
Just make sure the plug will leave adequate metal around the hole to be redrilled.
If the hole in not taped full depth of the threads, the tap not run in far enough, the partial thread depth will lock things tight when the plug is screwed in. Cut off about 1/8" high on each side an pean. Its not going to move no matter what once the plug is upset in the hole.
Though for casehardened parts I would rather weld the hole shut though with care brazed parts can be casehardened.
Low end MIGS like the Hobart "Handler" 135 I have are not that expensive and can serve many useful purposes and with some practice will make very nice welds. Faster than the other methods too.
Dan