Importance of that magnet for the running of the bike actually depends on which magnet broke. If it is a magnet that is on either end of the ignition coil when the points open then you will be producing a feeble spark and the bike might not run. If it is not them it is just the accessory coil will be a little week in part of making the electricity for lights, so you would have flickery lights. Depends on how much is broken off, you could have a bit more noticeable vibrations o the engine.
When the magnets are just about to leave their ful influence on the ignition coil is where you would have full saturation and the biggest potential for a bright blue snappy spark at all speeds. Less magnet on one end would produce less flux so less excitation to produce electricity.