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Negative ground is incorrect but won't make a dramatic difference in warm weather (some claim the difference shows up in cold weather as weaker spark because of the incorrect coil polarity -- I don't have enough experience to know). To change from negative to postive ground on a 2N:
1) Swap the battery cables
2) Repolarize the generator by momentarily connecting a wire from the negative terminal of the battery to the output terminal of the generator. That's it. Nothing else is required - but now on to the theory portion of why the starter doesn't care about polarity. Starters and generators both operate by the interplay of two magnetic fields. Imagine two bar magnets. When the North pole of magnet 1 is brought near the South pole of magnet 2, the two magnets attract each other. If you swap the polarity of the magnets, the South pole of magnet 1 will attract the North pole of magnet 2. In the starter, swapping the polarity of the input power changes the polarity of both magnetic fields. This results in the starter turning in the same direction regardless of polarity. This would also be true of the generator, except for one thing - residual magnetism. Generators begin developing their output power when spun by using whatever magnetic field is present in their field poles. The polarity of that residual magnetic field is determined by the polarity of the field during the generator's last operation. When you swap the polarity of the electrical system, the generator will still start up with the previous polarity. Momentarily connecting a wire from the battery to the output terminal of the generator (can be done by just jumping around the cut-out relay) will establish the correct magnetic field in the generator. (Note that this applies to the 1-wire 3-brush generator only -- other generators are polarized slightly differently.)
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