As you can see a spark, the main ignition problem areas should be that either the spark is occurring at the wrong time, or that the spark is too weak to fire the compressed mixture.
The first is unlikely unless the timing has 'slipped' (Paul's check on the distributor drive). You should be able to get a spark on number one, a centimeter or so before tdc (someone will know the correct distance). This will isolate timing.
If it is ok, you are left with the second issue - weak or intermittent spark.
The main things have already been suggested. You've replaced the coil. As suggested I would bypass the wiring. I had the experience of an ignition power wire work hardening from engine vibration and breaking inside the plastic insulation. Also, earth the motor directly to the battery - not via the frame. From there, it is a matter of checking / replacing the ignition components. With the exception of the capacitor, I would think that the electronics will either work properly or not at all.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.