Magnetics in motorcycle alternaters lose thier magnetic intensity over time due to the heat of being near the engine. The magnets of low power windings like model trains never get close to the temperature to start dislocating the magentic dipole moments. As for impact of a magnet at an accelerated rate losing the magnetic intensity, well that is a pretty low intensity magnet to begin with. (AKA a S#@tty magnet) Or one with an unstable chemical structure to begin with.
In the model aircraft hobby, we use motors with rare earth magnets. The magnets seem to last forever, UNLESS they are overheated. This can happen if you attempt to run a motor that can't turn freely for some reason, such as debris in the gears. This causes high current to be drawn which is dissipated as heat. As little as 140º F will effect the strength some rare earth magnets, though the better ones are good to at least 175º. A good rule of thumb is that if a motor is too hot to touch, it has exceeded 140º. Any loss of magnetic strength due to overheating is permanent. The magnets do not recover when they cool down. I know this from first-hand experience in building my own motors. Of course, really serious overheating can have other catastrophic consequences such as melting the insulation on the motor windings. - Jeff
