Actually the 10 amps will work if you use the Model A Voltage regulator that every car has as standard equipment!
Run with headlights turned on most of the time in daytime.
When battery was low turn off lights in day time.
This does not work if you have sealed beams though!
This was Very much like the Honda 150 which had a switch on handle bar to turn on lights.
Three positions:
1. Lights ON with Magneto charging for night.
2. Lights OFF with magneto off also, for day time so would not over-charge battery.
3. Lights OFF with magneto ON to charge a low battery during daytime.
All you had to do was remember to charge battery if it cranked slowly on start up.