Some time ago I was doing some work on an engine with a Webster Tri-Polar magneto. I found that the holes in the base for the mounting studs were stripped out. I repaired the holes in the following manner.
First, remove the magneto from the igniter. Then take off the springs and arm from that side of the magneto. Remove the four screws from the side plate, and then remove the armature. You will now have to take the other side plate off next. Carefully remove the coils so as not to damage them. The reason you might want to remove the coils? So you will not drill into them when you drill out the holes for the new mounting studs.
Use a No. 7 drill or a 3/16-inch drill, and drill new holes in the base where the old holes were. Drill the holes through the base. Next, use a 1/4 20 tap and tap the holes for the new studs.
Now you must make new studs. They will have to be longer than the old ones. You can make them out of threaded rod which you can get at most hardware stores. You can secure the studs in the base with Loc-Tite. You can also put nuts on the inside of the studs. You will have to grind the nuts to fit inside. I ground mine down to about half the thickness.
Do not remove the magnet from the magneto; you will lose some magnetism if you do so, and will weaken the magneto.