If you look at the connections in the light switch, only the park light is fed from the battery supply (brown wire), the other lighting is fed from the Y/R wire from the magneto coil, so the headlight supply is AC. That precludes the use of a DC-DC converter, unless you rectify it first - and with half-wave rectification you won't get the same power out of the magneto.

The coils on these magnetos are often would with a specific load in mind, so the terminal voltage is determined by that load - put a lower current bulb in the circuit and the terminal voltage will rise, though probably not by enough to successfully run a 12V bulb - but the total power available is likely limited by the magneto coil.

There's a separate winding (or maybe a tap) that goes through the rectifier/regulator to charge the battery. The regulator is typically a shunt type - essentially a large Zener diode that holds the maximum voltage down by dissipating the excess generated power as heat. One possibility might be to find an LED bulb you can power from this circuit, either directly or through a DC-DC converter if you're looking for more light output, but you're unlikely to be able to get more out of the magneto to get a brighter incandescent bulb. Running a DC-DC converter from the battery to power a 36W bulb will likely result in a flat battery.

If you look at the connections in the light switch, only the park light is fed from the battery supply (brown wire), the other lighting is fed from the Y/R wire from the magneto coil, so the headlight supply is AC. That precludes the use of a DC-DC converter, unless you rectify it first - and with half-wave rectification you won't get the same power out of the magneto.

The coils on these magnetos are often wound with a specific load in mind, so the terminal voltage is determined by that load - put a lower current bulb in the circuit and the terminal voltage will rise, though probably not by enough to successfully run a 12V bulb - but the total power available is likely limited by the magneto coil.

There's a separate winding (or maybe a tap) that goes through the rectifier/regulator to charge the battery. The regulator is typically a shunt type - essentially a large Zener diode that holds the maximum voltage down by dissipating the excess generated power as heat. One possibility might be to find an LED bulb you can power from this circuit, either directly or through a DC-DC converter if you're looking for more light output, but you're unlikely to be able to get more out of the magneto to get a brighter incandescent bulb. Running a DC-DC converter from the battery to power a 36W bulb will likely result in a flat battery.