No spark at coil.

Be certain you are measuring the voltage at the KEY side of the ballast and not the COIL side.

The "ignition run" (dark blue) coming out of the bulkhead connector supplies

the key side of the ballast

the blue field wire to the alternator

the "I" terminal on the regulator

the electric choke heater if used

So another point to check is the blue wire down at the alternator field.

Your voltage drop suspects are the bulkhead connector, the connector on the ignition switch, the switch itself, problems at the ammeter or the "in harness splice" which supplies the ignition switch, or the battery/ alternator infeed wires, again, coming through the bulkhead connector