Take a trip to Jaycar and buy a suitable AC charger.
Open the box and bypass the transformer section with a couple of shunts so the AC supply feeds directly to rectifier section of the power supply.
Remove the transformer if you want to save a ffew more grams.
Cut the AC plug off and attach to alternator output.
For higher output like LED headlights you may need to look at beefier rectifier bridges. There is likely a Kit with high enough capacity diodes to handle the current draw.
My bike is slow but the earth is patient.