The amp meter has nothing to do with the regulator. It is really a sensitive voltmeter that is measuring voltage drop across a piece of wire in the wiring harness from the starting solenoid. To check that the meter and wiring is good remove a red single wire from the battery side of the starting soleniod, it may have a yellow colored ring connector, momentarily touch a AA battery between this red wire and a thick black wire that was on the same post. The amp meter should swing one way. Reverse the battery and it should go the other.
Ok that is easy enough. If the gauge is not good? Where would I find one of them? I have received a catalog from AutoKrafters and one for Johns Cougars in Michigan but neither has any gauge's for sale.
The amp guage in a '67 - 68 XR-7 is a shunted unit, so very little current actually goes through the meter. The meters lose their sensitivity over time, and the only way to get it to function is to replace it with a NOS unit.
Really, a volt meter is a better solution as it gives better, more relevant information. The RKTMN can fix you up with a nice unit that looks like it belongs in your dash.
I have an NOS original amp guage available for 67-68 XR-7. Send me a PM if interested.