After Market Ammeter?

Ammeters work fine and always have. They show charge into/out of the battery in this setup (except for the cranking current); not current out of the alternator. Voltmeters are better for most people because it is easier for most people to interpret the indications. But they have limitations in what troubles they can indicate too. And buying a decent voltmeter is a lot cheaper than a good quality ammeter.

FWIW: My '62 car's ammeter is in good shape, connections and all, and will stay in as the car is so original.