There are many ways to fix this broken mag plate. Dales idea is a good one and does not cost much.
If this motor were mine I would have the broken piece professionally Welded.
I would then install a thick washer in slot to prevent over clamping force from
Being applied.
To provide correct friction for motor operation, I would use very thin brass
Shim stock between surfaces to provide friction to prevent unwanted mag
Plate movement.
