Why not just cast up repair sections and offer a service to get the new section welded into the corroded out cases (probably need some final machining). Would be cheaper for you and the customer.
Not a chance, first up aluminium and magnesium castings are like chalk and cheese
You can’t weld mag with aluminium rods and you can’t weld aluminium with mag rods so you would never get the 2 parts to mate.
Then you would need to cut out a section of the old case to fit the new part giving you alignment problems of the shaft.
Not to mention distortion problems from the heat.
Those Mag cases are a pain to work with at the best of times, much much easier to just bolt on a new part and go riding.
The Hondas I would consider great bikes for their era and are worth spending the money on.