Well, after six months roller-coaster of hope / failure today I tried to start the project with its beefed-up starter and sprag clutch replacing the ratchet. Works a treat. Been such a long haul it's actually difficult to feel elated about it. Relieved, certainly.
Now I need to convert the bike to 12v as so far I've been running the bike on its 6v electrics whilst using an external battery for the starter.
Question - I fitted a modern combi switch (lights / horn / indicators) for economy of space reasons. However, I noticed a voltage drop with the lights. Putting it down to 2x normal amperage draw due to using 6v, I fitted relays. When changing to 12v (half the amperage) will I still need relays - ie, do modern bikes have relays for lights? The wiring associated with the switch (about 10 wires) does seem flimsy.