Afternoon Rusted,
You have a few of options, try and find another set of cases from any year, try and find a set of 53' cases to match your frame or get the ones you have repaired.
The four bolt holes in the sump are easy, a competant welder will be able to weld them up and then they can be drilled and threaded.
The maggy mount isnt so bad either, you will need the servces of a machine shop. The cases need to have the outer face milled flat and then turned over and the maggy mounting face also milled flat. This will allow the maggy to sit slightly further forward but a few thou wont matter. None of this is hard and any machine shop should be able to sort it for you. About the only thing you may need to watch for is when you reassemble it the gear that the generater sprocket goes on has clearance in the case, it only needs to be able to turn freely, it doesnt need any end float.