The Lucas K2F magneto uses one set of contact points to produces the sparks, which are then fed to the cylinders alternately, by the slip ring on the armature. In other words the plugs do not fire together, as on some other ignition systems.
So the fact that the bike runs on one cylinder means the spark generating part of the system is working, and the fault is in the distribution of the spark. Suspect the slip ring, and then the pick up/brush, plug lead, cap and spark plug of the dead cylinder. A mechanical fault on this cylinder is also a possible, but for now try this.
Inspect the magneto slip ring and pick up carbon brushes. They all need to be clean, brushes are spring loaded and move freely. Morris is on the money here.
Still no better? Change the plugs over. If the fault also changes sides, it's the plug. If the good cylinder still runs, the plugs are OK. Similarly swap the leads and plug caps. If the result is that the poor cylinder still won't fire, then suspect the pick up and its brush, plug lead and cap. The old trick of holding the plug lead close to the cylinder head with the engine running lets you see a spark jump on the dead cylinder if all is in order. Hold the lead with a piece of wood or insulated pliers or wedge between the fins. A good spark from the end of the lead and it still won't fire the plug indicates a plug, plug cap or mechanical problem.
Swarfy.
