The way these motors produced spark is pretty simple. 12VDC energized a coil wrapped around an iron core. The points, driven by the little cam in the distributor’s shaft, were in series with the coil primary and opened the circuit when the distributor cam opened the points. The magnetic field that had developed in the coil’s coil collapsed when the circuit was opened and induced a voltage spike in the iron core. The induced spike was then directed to the proper sparkplug by the rotor. There are some minor details like the condenser (a capacitor across the points to prevent charges and material transfer), but that’s basically all there was to it.
Since the rotor is turning when you try to start it, you have correctly assumed that the timing chain is good. You’ve also verified that you have no mechanical failure between the camshaft distributor gear and the distributor.
Since the problem seems to be electrical electrical, you’re going to want to start by checking the coil primary for voltage. It’ll be VDC, basically a squarewave. If you’re using a simple voltmeter, you may see it as a low voltage. You’ll actually be reading the average between the full battery voltage and zero. If you’re using a scope, you’ll be able to see the actual wave and check the waveform.
If you have a proper waveform on the primary, and nothing out the core, the coil is fried. The primary is being turned off & on, but you’re getting no induction, probably because the coil windings are shorted.
If you have full battery voltage, than the coil is shorted to ground internally and you’re getting no induction to the primary.
If you have zero voltage, than either you have an open or a short to ground in the wire leading to the coil. Trace back to the starter relay, and to the battery if necessary, from there.
One more thing: be sure your ground straps from the engine to case ground are good. If they aren’t than they might have enough resistance to eat up (“drop”) too much of the voltage to allow the coil to induce sufficient voltage to spark with the added large load of the starter motor, but still allow the starter motor to crank.
Two more thing: be sure the points are opening as the rotor approaches one of the spark plug paths; look at the posts under the cap, turn the crankshaft by hand until the points open, and see if it’s lined up with a sparkplug connection under the cap. That’ll at least give you an idea if things appear normal without pulling the timing chain cover off. The points need to open and induce the spike in synch with a sparkplug connection at the rotor, else the motor won’t fire.
Post back with any questions or with the results.