There is only 1 field. There is no way to only flash half of it.
There are only 4 things that will cause this set to not produce electricity:
1. Loss of residual magnetism. This is what flashing the field fixes. There is another way to flash the field, if you want to try it. Simply apply 12V across the terminals of the capacitor for 2-3 seconds.
2. Bad rectifier (diodes). Testing them is simple, but I would need you to get a picture of them so I can determine which type they are.
3. Open winding in the rotor (field winding). This can be tested with an ohmmeter.
4. Open winding in the stator.
To test the field winding, disconnect one lead coming off the rotor. Set your meter to ohms and place one lead against each rotor lead. What is the resistance?
To test the main windings, disconnect the 4 large leads that come out of the stator.
There will be 1 ground, 1 neutral and 2 hots.
Set your meter to ohms.
Test across the following wires:
Neutral to hot 1.
Neutral to hot 2.
Neutral to generator frame.
Hot 1 to hot 2.
Hot 1 to generator frame.
What are the resistances of all of these?