Remove electronics board

Mind the two lamps on the fronts, put them aside. 

Pull contacts sidewards to release PCB board. Take PCB board off.  

(image of other model shown here)

Replace motor and worms

Remove the worms from the original motor (a gear puller, available here, comes in handy, but they should pull of quite easily by hand).

Solder wires to the motor

There is a round mark next to the plus pole of the motor. This is traditionally a red wire. Solder a black wire to the other connector. You can use any colour you like, but as long as you remember which is plus and which is minus. When you install the motor in the train chassis, make sure the connectors are not touching the frame.

Check fitting

Test the brass adapters to fit on the shaft. They have different lengths, where the longest one fits on the longest shaft (the side with the contacts). Fit each worm on the end of the brass adapter tubes and slide this adapter tube on the motor, not glueing it yet.

Test

Put it in the model, to make yourself familiar with the location of the adapter tube. Then super glue on the shaft, leaving about a millimeter of space between the motor and the shaft.