I believe the ballast resister was on the back of the ignition switch (look for a cylindrical wire wound resister between two terminals) It could also be a ceramic device down on the side of the distributer. If the resister is open or the circuit from it to the points isn't good, then the only time it will fire the plugs will be while the starter solenoid ignition terminal is supplying full 12 volts during cranking.
Also may have to adjust the points a bit if the voltage drop across the resister makes the spark too weak once you release the starter.
What have you got the sparkplugs gapped at? Might be over 0.020 which is about all the one we have wants to fire across.
Over the years we have battled with a LPG 3020 and finally have gotten a high voltage aftermarket coil that seems to be doing the job. Before that we had bypassed the ballast resister and let it have full voltage all the time, but that's hard on points and the standard 6 volt coil.
It has both a starter pushbutton and an ignition switch with a "start" position. Also before the high output coil, (and the resister bypass) you could hold the key switch in the start position as you cranked the engine with the pushbutton and continue holding it until the engine sped up and leveled out or it would die.