6v did not need/use a ballast resistor (flux capacitor)*
A change to 12v requires - a 12v coil and a ballast resistor (preferably); or, - a 12v coil with an internal resistor, which keeps the points from burning, but, does not allow the starting "advantage" of the 12v-coil/ballast resistor set-up.
* an adjustable flux capacitor must only be adjusted with with clutch-head sonic screwdriver.
