First question. points and condenser or electronic ignition? If it is a two wire ballast then points/condenser system if 4 post then electronic ignition. There are TWO separate paths the key provides... One for "run" and one for "start" plus start has the additional lead to the starter relay to engage the starter. That obviously works, and so does "run".
During "start" power is supposed to go to the ballast resistor which then feeds the Coil directly (for hotter spark during starting ) AND thru the ballast resistor to the junction that feeds the Voltage regulator, the electric choke, AND the ECU on electronic systems. If that circuit is open or LOW on voltage you will not get a spark!! Path for the Start circuit is from key, thru connector on steering column, thru the firewall connector (pin 22) then to the ballast resistor (same post on ballast that has the brown wire that goes to the coil). Test using a multimeter on voltage at that post while having a buddy turn the key to start. If NO voltage then broken wire or loose connection along that path. If it has voltage but not at least 10+ volts then again problem in that circuit!! If voltage much less than that, and is electronic ignition, then the voltage on other end of the ballast which feeds all the system will be below 6 volts and the ECU will NOT function and the coil will not fire. You can check that voltage with a muli-meter on the other end of the ballast while cranking.
PS...that circuit needs to have strong connections because when the power runs that direction thru the ballast resistor it drops the voltage by about 40% before it gets thru to the ecu. On RUN the power to the ECU does not go thru the ballast and thus can be weaker.
Hope you have already solved the problem, or this helps !