You would need a 12 volt regulator (quality one may be hard to find and pricey) and either a direct 12 volt coil "no resistor required" (NAPA IC14SB or equivalent) or a ballast resistor between switch and coil.
You would not need to change battery ground with a generator, but make sure regulator matches battery ground. If positive ground the + on coil goes to distributor, if negative ground the - on coil goes to distributor. Polarize generator before starting engine.
If you switch from positive to negative ground be sure regulator is for negative ground, polarize generator, connect coil as above, and check ammeter if it has one. With switch on, points closed, and engine not running the ammeter should show negative. If it is positive switch ammeter connections.
Points, condenser do not care if 6 or 12 volts or positive or negative ground
6 volt starter should work on 12 volts and will turn same direction regardless of battery polarity