1962 Dommie 99 rewire

 Suitable looms are availiable , I thought them too flimsy and decided to repair the orriginal (which is still in use) .  I would fit an extra earth wire up to the headlight shell and  a good heat sink to both Zener and rectifier .  I also run with the pilot on to keep the zener load down.

I think all bikes (especially 6 volts like my two Nortons!) should have dedicated earth wires to its extremities.  Tail lights and headlight.  Use a domestic, nickel-plated earth terminal block behind the battery box or somewhere close to the battery, but out of sight.  Take a fat earth cable to it and then run the cables wherever you can.  

The distributor could benefit as well as the horn but the lights especially will be better without resistance losses due to weathered contacts and terminals.

You don't say what year what model? Of course knowing such and quoting to wiring company might give you trouble if the bike is supposed to have coil ignition.

I can offer you an alternative, I sell a wiring kit for alternator bikes, so you just rewire what you have in your way. While you are modifying a loom you can have wired the bike properly. Also a Magneto with a Zener diode is an unsafe situation-facts are on my web site aoservices.co.uk  12V Alternator and magneto-how not to do it. As well as the loom kit I can help you with any specific problems you run into.