Cruisertrash, Most cores are going to be the original ND case/guts and then rebuilt(bearings, brushes, regulator) by some of these aftermarket companies. There might be some knock offs that are new(from china), but for the most part, it is an original ND, but not reman'd by ND like they used to be. They might be using high quality parts for the rebuild or cheap parts. Yes, Bosch typically does a good job(at least for the original applications), but even they do not rebuild like ND does(at least in my experience).
Abe, best to ask the shop(D&D 303-423-6522) In theory, most shops can if they have the right tools. And then the the desire to do the tedious job of rewinding the rotor, dipping the assembly and baking the assembly. Then have the right size voltage regulator to fit the case(unless it is external VR).
Then wanting to warranty the product.
Doug has stains in his fingers from doing this job for a very long time. He is a master of the trade, and could do many things most people can not.
Doug has lots of cool old Generators, Alternators, starters 12V, 24V 6V stuff on the shelf he has rebuilt just waiting for the right customer to come thru the door. He may have what you want on the shelf already built.
Doug shop is a throw back to a long time ago, very similar to Vanetta. Vennetta has been gone about 3-4 years now. Boulder raised the
business property tax by 30% and many little mom and pops places closed up. He could not afford the rent any more. Shop had been there
since the mid 50's. He told me he might start up at his house, but never heard from him. He lives in Lyons area.