A good quality voltage regulator??

[rotorwrench]

I see old originals on flea-pay now and then. They were rebuildable back in those days. A person would need to find a good auto electric shop to have it tested prior to use. They were also made to be adjusted more easily than any of the new stuff. It's well worth looking around. NOS parts may be out there but I would still have one checked out before trying to use it. The ones with the extra ground terminal are getting harder to find and can get pricey.