12 volt neg ground on a B N

Get a distributor with a 12 volt coil that states "no external resistor required", and connect 12 Volts from your ignition switch to the (+) post on the coil. Connect the (-) post on the coil to the points terminal on the distributor.

Alternately, if you don't want to spend the money for a distributor, and have an IH magneto with the external coil wire from the upper coil terminal to the center terminal on the cap where the sparkplug wires go, you can get a 12 Volt coil, as above, connect the wire from your ignition switch to the (+) post on the coil, the (-) post one the coil to the brass nut terminal on the mag where the wire used to go to ground out the mag, and modify the mag as follows...

Inside the mag, remove the internal condensor, and disconnect the wire from the internal mag coil from the points, and tape it up.
Use the appropriate sized machine screw to attach the lead from the points to the brass terminal that passes through the shell of the mag, that you now have the (-) wire from your coil attached to. Obtain a condensor for the same application as the 12 Volt coil you are using, and attach it to the outside of the nagneto, and connect the condensor lead to the brass terminal on the mag, where the wire from the (-) side of the coil is attached. Ad a high-tension coil wire from the center post of your 12 Volt coil to the center post of the cap on your mag, where the sparkplug wires attach, and your mag will now function as a 12 Volt distributor. (Quite well, I might ad.)

Hint... to avoid having to time the little gears in the mag, do not remove the flat cover under the mag cap that the rotor shaft sticks through. Instead, remove the three screws that fasten the complete piece where the mag distributor cap attaches, and remove the whole end of the mag as an assembly. The assembly will pull off of the main shaft in the mag, which has a flat on it, so it will only go back together one way.