Swift and dirty.........
Pull the air cleaner off, start engine, as it dies squirt a little petrol from a washing up liquid bottle (or similar) into the intake.
If you can keep it running (think of those YouTube clips of guys starting big block motors with no chokes) then fuel is the issue.
Check the filter is not blocked by running without it, ditto the exhaust.
If you don't have a compression tester stick your thumb over the spark plug hole (with the plug out!) and get some one to rotate the crank, is there a difference between cold and hot, after the engine gives up?
After that it's electrics, first check the timing of the points. If that's spot on then out of the 'come in handy box' rig a circuit through the points with a 12v coil, 12 volt battery and a condenser, thus bypassing everything but the points, if that works it's likely to be the magneto or the condenser.
Do it it that order, least amount of work first......
Cheers
Stef