It may be something as simple as the dynamo has lost its residual magnetism. Try 'flashing' the dynamo with the engine running briskly and touch one end of a wire from the battery connection on the starter motor and the other end on the 'F' terminal briefly. you should notice the cutout points closing. Remove the wire and rev the engine a little more and you should be rewarded with a charge showing on the ammeter. The ignition light should then go out. It will take less time to carry this out than it has taken me to type this!
Good luck
Cheers
Bob
Good luck
Cheers
Bob