If a generator will motor and start the engine your generator is good. You have to look at the charging circuit now, not the starting circuit.
Sounds like your voltage regulator is not closing, could of lost its polarity.
Take a 12 volt test light with the engine off. Clip you lead onto the "A" terminal of the generator and touch the test light on the positive terminal of the battery. It should light up. Now do the same test with the engine running and the light should go out. The battery is putting out 12 volts and if the generator is charging 12-14 volts there is very little current flowing thru the test light and not enough to light it up. This means the generator is good. What you just did is make an idiot light out of your test light.
If it were mine and the generator tested good I would pull the cover off the regulator and start the engine and lightly push on the relay(s) to see if the cutout relay stays closed and see if it charges. The cutout relay is the one inline with the "A" terminal on the regulator.
These are just a couple of the simpliest test on a generator circuit. There could be many other problems such as a bad ground to frame with the regulator, bad connection on the amp meter circuit, the field winding in the regulator not closing or resistor is burned up on the bottom side of the regulator.
One final test is to ground the "F" terminal on the generator while the engine is running and this will by pass the voltage regulator and cause the generator to charge full throttle. Now the cutout relay will have to close for this test to show a charge. The cutout relay is the "switch" sort of speak between the generator and the charging circuit. It needs to be closed when running to charge and it needs to be open when the engine is off to keep the electricity from back feeding and draining the battery.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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