karmann54,
It only takes a very small voltage drop to the coil to make the 6 volt electronic units stop working. It would be a good idea to run a wire direct from the battery to a relay next to the coil, then from relay to coil, using the original coil positive wire to trigger the relay.
Normally, main wire from battery goes up to headlight switch, then to ignition switch then back down ignition circuit to coil. All this means lots of places to get a drop in voltage and when you're only starting with 6 there's not very far to go.
😂 Some people swap the wires so, main goes to ignition switch first, then to headlight switch But this can also leave you with poor/er headlights. On a 6 volter you really need clean connections everywhere and use relays wherever you can. Dielectric grease is good to put on clean connections to stop them corroding. You can get some at supercheap and so on.
You could also fit a hard start relay to prevent voltage drops when using the starter motor. It will also give better voltage to the coil. This might be enough on it's own, or maybe, you could do both.
Wiring diagram for hard start relay (on page two):
http://www.oldvolkshome.com/hardstart.pdf
Regards,
Matt.