Putting a new power cord on an Erector motor is straightforward. The leads on the cords are soldere to the leads inside and then wrapped with electrical tape. Getting off the old friction tape will usually require some work with an exacto knife or equivalent as it is often pretty dry and brittle with age. The main challenge is that there is not a lot a lot of wiggle room in any of the motor cases. So placement on reassembly has to done carefully. One other thing is that in removing the rotor, be careful not to lose the small insulating washer spacers on the axle.

THe electromagnets are not as difficult as you surmise. The earlier large magnet cases are held on with a screw and small double angle. remove the screw and the actual magnet assembly is removed by pulling the rotted wired through the hole in the case. The smaller red magnets have a round hook at center outside the case, but it is actually a screw end. Unscrew it and the case and magnet pole come apart the same way. the two wire leads are soldered to the ends of the lacquered wire on the windings. Remove the old leads and replace them with similar gauge mew wire leads which will pass through the 2 holes at the top of the magnet insulator, resolder to the winding wire. But if there is a break anywhere in the winding, you would have to rewind the magnet. In that case you could the number of turns in removing the winding and then rewind with the same number. And if you are off a bit with the magnet, it is not as critical as it would be rewinding a motor pole.