Alternators would have been a pretty new concept in 65, most other manufacturers were still using generators during that time frame. I would suspect a modern alternator and a matching regulator would probably provide as much (probably more) power as that heavy duty 65 version.
Unless your going for the date perfection thing, I'd just go modern, finding something that may well have been an after market product to begin with, or a 1 or 2 year optional part production may prove to be extremely hard these days. Gene