Adding a photo of my "taper punch" repair of a Gran turismo stop plate with stripped-out flats at the hole.
A few whacks of a hammer and punch to move some metal over, followed by some dressing with a dainty jeweler's file, restored the proper shape and function:


I recall that in some cases, the position of some Simplex derailer's stop plate can be secured by simply tightening the back-side mounting bolt (once the stop plate has been forcibly rotated to a correct-tension position). Not as easy as merely turning an Allen wrench to adjust the tension, but does secure the needed B-pivot spring tension.
Something like a needle-nose Vise-Grip might be handy for this (to grip and turn the stop plate).