How you proceed depends on what's best for you. As an observation, an existing commercially available loom will cope with 12 volts. The current flowing remains more or less the same whether the system is 6 or 12 volts. The higher power required by higher wattage bulbs is compensated for by the higher operating voltage, and 12 volts gives a little more choice in the way of bulbs, horns and electrical accessories.
The relative cost differential of 6 volt and 12 volt armatures and field coils is marginal, and if the existing dynamo is truly finished, then well worth swapping for the higher output parts.
The usual practice of wiring brake and horn directly prevents the ammeter being subjected to irregular high currents, more as a way of preserving its longevity, and also if wired through the ammeter a failure would render these "safety critical" components inoperative.
The Lucas regulator terminal block always raise a smile..... depending on model the letters are D.E.A.F or F.A.D.E Says all you need to know about Lucas Electrics......
If making your own loom, add a nice big earth wire from Forks to frame/battery earth. Normal earth return from the headlamp and forks is through the nicely greased headstock bearings, and adding an extra earth often improves an otherwise weak headlight output. Never been a fan of magneto cooking, preferring airing cupboard or top of the wardrobe storage
Swarfy.
