In looking at the pics of the motor, it looks like you have a Pertronix ignition coil on it. It also looks like the coil wiring goes through the ballast. I wonder if you have a coil that is already set at say 1.5 ohms, and adding the fact you have the ballast which reduces the voltage, you might not be getting enough juice to spark? Have you looked under the distributor cap to see if the previous owner updated to a pertronix ignition? On that coil the part # should be present, go to pertronix's website and see what you have. Just something to think about, in addition to all of the other's tips for you.

I have found out too that Pertronix does not like the lower voltage. An easy thing to check. Also make sure they are not using the coil hot wire to operate an electric choke. This will pull voltage away from the coil.