Since there's a non-original charging system involved, it's probably going to be necessary to design your own circuit. Look up the term "Wheatstone Bridge" and see if you think it would be possible to use the normal voltage drop from the alternator to the battery to provide a reference voltage to swing the ammeter needle from charge to discharge. It's been close to 50 years since I studied any kind of theoretical electronics like that, but it seems that would be the sort of circuit you'll need to use.
Jerry

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