6V Flywheel Magneto

I'm just beginning in electronics, but from what I have seen so far, a step up transformer should change six volts to twelve volts. You just get half the amps.

I'd steer clear of it unless you work in electronics and have access to the wire, other components and expertise.
Many years ago, I rewired my old Lambretta and simplified it right down to just a few wires, I ended up rewinding one of the magneto coils to cure an intermittent ignition problem.
Automotive electrics are simply pants.
Del

I have an idea. But must be approved by experiment.

Since the voltage, you obtain from your magneto is 6 Volt AC, you can rectify it with a voltage doubler and charge a pair of 6 Volt small 4Ah. gel batteries.

As you can see in the picture, each battery will be charged simultaneously with +6 and -6 Volts. And the total voltage in the serial connected batteries will be 12 Volts.

First try if you are able to charge the batteries and can obtain 12 Volts at the right hand output. If succesful, then disconnect the negative (Chassis) side of the lamps of your bikes and led them through with a separate cable to the negative terminal of the battery bank. (to -6 Volts) And remove the live (Switch) sides of the lamp connections to the +6 side of the battery bank. Pack the batteries by binding them with an adhesive tape to be as one casing.

Hope this will work.

Regards