The free black lead can be soldered onto the central earth post. But it needs a large soldering iron - 15W isn't enough. A 40W iron will do it. Alternatively, a brass or copper strip on the chassis under the motor can be fashioned. Ensure there is point where a wire can be soldered, and put a crease in it so that the motor will press down onto it to make a decent contact. Some versions of these locos had such a strip soldered to the negative motor terminal to connect it direct to the chassis. If so, this should have been removed and can be used for the black lead. Alternatively, when the motor is in position, wedge a bit of card under the motor to keep the strip in place.

The white lead can be soldered to the front light tag. Take off the red insulators first and have the bare end pointing upwards when soldering so that the insulated part is at the bottom - as shown in the photo. Put the bulb in place and reassemble. The white lead goes over the top of the screw heads. Check out the underside of the boiler weight. There is a groove down the centre to make room for the white wire.