The first thing to do is find out which cylinder is running and which is not this can be done by starting the engine and pulling off one of the plug leads, if the engine still runs then that is the cylinder that is dodgy then swap over the plug leads then restart the engine and pull off the same side lead if the engine stops then you have a dodgy lead or plug cap. If it continues to run then both leads are OK and you can go on from there. You do not say if you have standard points ignition or electronic ignition as this will make a difference as to what you should do next you could try fitting a new set of plugs I have a similar bike an A65 and have found that plugs can spark OK out of the bike but fail when running even nearly new plugs can fail.
