I have a couple of N2's with the same design of motor, an open frame 5 pole unit. It is noisier than the can motors found in more recent RTR products but it is very robust, and easily converted to DCC control. You need to isolate the motor brushes from the track supply, and interpose the decoder. Judging by the diagram the uppermost brush is fed by a wire from the pick up wipers one side, probably soldered to the brush terminal so easily disconnected. The brush on the underside of the motor may well be fed by an extension from the pick up wipers on the other side, simply sprung against this brush terminal. To get at this, the motor will have to be removed, the securing point will be on the underside, a cross head screw centrally into the pole piece. The motor may well be a tight fit into the moulded frame in which it is seated, which may also surround the front bearing of the motor. A ltttle wiggling should free things up. With the motor out the spring contact can be bent clear and shaped conveniently to be soldered to one of the rail inputs on the decoder, and one of the motor output wires on the decoder soldered to the underside brush terminal before the motor is reseated and the securing screw replaced. Use a 1A decoder as this motor can draw a fair current, and if you want good performance a high grade decoder pays off.