Sounds like the problem might be with your generator and not your amp meter. You can remove your amp meter and test it on the workbench using a six volt battery and a current draw like a six volt fan motor. The bench test will determine if your amp meter is working correctly or not.
If the comm. on the new armature is slightly rough that will cause the brushes in the generator to bounce, therefore the needle on the amp gauge will also oscillate back and forth. The brushes must be seated with a brush stone or the amp meter needle might tend to oscillate. A weak brush arm spring will cause the same problem.
To remove the pole shoe screws, place a long round metal bar the size of the screw head on top of the screw and hit the metal bar hard several times with a heavy hammer. Then use a impact screwdriver to remove the pole shoe screws.
If the armature is definitely fried, then I would replace the field coils as well as an added precaution, and when you attach the wires inside of the generator make sure that you attach the wires correctly.
