How to re-magnetize a permenent magnet motor of small size (model railroad)?
I am not an expert here, only having done this once or twice a long time ago, but the "usual" method is to send a very short pulse of high current (e.g. >10A, &lt;1us) through the winding, in order to saturate the magnet completely.
All you need is a large capacitor bank and a switch (plus a frame/shield to hold in in place in case anything decides it wishes to depart in a rapid fashion from the motor) You can work out the voltage needed to charge to by measuring the resistance of the windings (roughly, as it will be a fast rising pulse as opposed to DC you are sending through, but it will give you some idea)

I had a quick Google and found this page which discusses the method. I suggest you try it out on and old motor or magnet first, as it is likely to be a rather empirically judged process. This document looks interesting and useful also.