If you have a Lucas dizzy with a vacuum advance then as best I can remember, another possible issue may be the very thin,usually green braid insulated flex that ensures the continuity from the top movable plate on which the points are fixed to the fixed plate that that is screwed to the body of the dizzy... This flex can break internally making for an intermittent sparks. While you are in there if you remove the rotor arm and put a drop of oil on to the centre of the cam lobe to lubricate the shaft on which the bob weights and springs mounted below operate the centrifugal advance mechanism.... That mechanism may well be seized, though would not be a cause of intermittent sparks but could affect how well it might run on the road... Rather than complicate matters further a this stage with ideas on how to free the mechanism off (if seized) , a drop of light oil might help a bit at this stage of the game.
Also hope you have fresh fuel available at the carb, and have replaced/cleaned any fuel filtration in the system..? A spray can of Easy Start or similar might help the initial fire up process, if used sparingly.
Good luck
