Too much electricity!!!

Get a regulator/rectifier that can handle more watts, you can get 200W capacity ones for Honda's for peanuts.

Heated waistcoat.

There are 6v zeners around nowadays for scooters , or convert to 12v and regulate. Conversion is cheap as your alternator is unchanged as long as the LEDs can take 12V, the ones I use on my Commando work down to 5V which is their internal voltage behind the integrated voltage regulator inside them.

The first thing to try Harry is to disconnect the "High Power" set of coils from charging the circuit. Assuming you have a standard Lucas Rm 13-15 alternator you are likely to have 3x wires that are connected to the wiring loom near the chain-case with a 3 way bullet connector.

You need to remove the GREEN/YELLOW (alternator side) wire from the connector and insulate the end either with tape or simply slipping the bullet into one end of a single bullet connector to insulate it.

This will leave the common wire (white/green) and low power (green black) still feeding the rectifier. Try this as it costs nothing and can be done in seconds.

If the battery still overcharges, fit a 6V Zener diode made for the job it should fully clip the output at around 7.2V to fully charge the 6V battery.

Les

You could always use a power sink - say a high resistance bit of tungsten in an evacuated glass sphere - and use the resultant bright glow to illuminate your progress as you ride along. Perhaps I should patent the idea.