Disconnect the leads going to the brushes, and check for continuity (resistance) of the field winding. You should be able to measure *something* on the order of a few ohms. If you have resistance, then check each brush contact to the shaft that the field winding is on. There should not be any resistance at all, if there is, you have a shorted field winding and will need to be replaced.
Check the resistance of the stator winding next, make sure that nothing is connected to the generator, disconnect the stator wires if you are able (label them!). Check for resistance between the leads (2 wires for a 2 pole, 4 wires for a 4 pole). If there is no resistance, then the stator is open, and will need to be replaced. Also check for resistance from each wire to the frame of the alternator, there should not be any resistance here (indicating a shorted stator). If there is, then you have a shorted stator, and will need to have it replaced.
Replacement is likely to be impractical, probably cheaper to buy another generator with the same engine and keep the old engine as a spare. Still, contact an automotive starter / alternator shop for prices.
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