Hi Mike,
I would suggest that you have an incorrect coil. It is entirely possible that although the coil is for a 12 volt system, it could be a ballast resistor type used on a modern vehicle, possibly only designed to have 9 volts or so running through it. I have come across this a few times. If you do go down the electronic ignition route, then fit a new coil that is recommended. I have fitted Powerspark kits on a number of vehicles and they transform the running and starting.