Sounds one maybe for Ron Brown.
Your light bulbs on the 1977 would all be 6 volt. It could also affect your High tension coil unless you swap out the 6 volt coil for a 12v.
The diode does control the flow of current to be in one direction but it's main function is to convert AC (from the magneto) to DC (to trickle charge the battery when the engine is running)
Are you thinking about swapping engines? Does the 12 volt have a CDI ignition?