Well, there's always a way to save this old stuff if your pockets are deep enough.
A good machine shop that has a CNC horizontal mill would be a good place to start asking questions about removing the gear. They should be able to use circular interpolation ( computer guides milling cutter in a circular path ) to cut away the weld.
I don't think a large lathe is going to be an option with the flywheels still mounted and in the way.
If you are going to talk to a shop, do it BEFORE you start torching away at the gear. Sometimes torching can leave metal harder than hell . Ask them if you can save them time by torching first.
As to the taper pin, I may be able to give better advice if I knew where it was located . I can offer that they are case hardened and can be drilled after you get past the hard outer skin.