Great locomotive, I have a 682 in my collection same thing as 681 but with the oiler linkage.
You've been recommended to Olsens and I'd do that first. These are pretty robust but still you need to watch what you do in them. The motors have many small parts that are easy to overlook when disassembling like the use a small bearing ball on the back end of the motor for thrust reaction on the commutator. You'll find that motor removal to allow cleaning and lubrication of the primary gear reduction will do much to reduce the amp load and resultant motor heat which will allow more cars to be pulled.
I wouldn't rewire anything if it isn't needed; not because I'm a purest but these solder connections are to what can be delicate parts and the less they are disturbed with heat the better for them.
Bogie
