In addition to what has been mentioned, I would try substituting a coil from a newer vehicle such as a 1954 Chevrolet with a condenser on the negative lead and no resistor. The original coils are known to be unreliable. They have the condenser built-in and unless it has been taken apart, as in having the tar melted out and condenser replaced with a modern equivalent, it's definitely bad by now.
Another question, do you have the carburetor intake hooked up to the pre-heater on the exhaust? These motors run best with warm air.