WOW, great advice from all above! I will repeat some of it. You have not described any sounds/actions/speed of motor when trying to start. If you really do still have very good spark at plugs it would seem unnecessary to check electrical components. Retest spark.
If you still have good spark, squirt/dribble gas down carb then try start right away. Be sure choke is not stuck on. If it fires with that fresh gas...BIG clue.
When you detect some change somewhere when trying to start, something different happens , something strange intermittently....that will be your hint. Keep aware and observe. The abruptness does sound like a wire fell off/hanging by a thread/weak connection or electrical component failure... IF fuel down carb does not result.
When you say coil is good and this and that is good, we assume you know how to check these things properly and are not mistaken.
Need to know if no start means: absolutely NO cylinders firing OR trying to start with some fire? Any backfire at carb or tailpipe? Any air noise from carb? take of air cleaner to hear. Does motor sound like normal starting compression or faster than normal or fast/slow/fast/slow? Be sure to keep the battery charged up. Suggestions may change when you supply this info and if gas down carb had any result whatsoever.
No result? Recheck:
1. Coil
2. Points AND Condenser
3. Timing /plug wires vs Firing order on dizzy
4. Ballast or bypass system break down
5. Ignition switch failure or wiring
6. Grounding failure somewhere. Grounds will fail under load of starter just like power will.
