If the ammeter needle does NOT move when you turn on the headlights, as I think I understand from you, the ammeter is not connected properly or the ammeter is defective.
Start there.
The diode cutout might also be bad, but you need to get the ammeter working first.
Trace the wires from the generator to the terminal box to the ammeter to the terminal box to the starter switch. Something in there is not right.
When you turn the headlight switch on, do the lights light up? If so, the headlight switch is getting power from somewhere. If not, that's why the ammeter needle is not moving.
If wired correctly, the ammeter might just be defective. Is it an original or a reproduction? The repro ammeters are notorious for being defective.
