I have 3 of these locos.
Having got the body off, remove the motor, by undoing the screw at the back of the motor (not the long which goes through the magnet but behind that).
Slide the motor out, there is a spring clip which holds the brushes in place about one inch long running from the magnet to the brushes. One side of this clip should have an isolating tube so that the metal of he clip does not touch one of the brushes (the one with the wire attached). The other side is exposed and touches the other brush. If its mot like this thn you found the problem.
If it is like this, apply power to each side of the motor. It should run fine. If not, disconnect the motor from the chassis and run again (note it should turn easily by hand). if it still not work, there are companies that will do a rewind. If that is too expensive, don,t through it away sell it on eBay as a dud, people will buy these to repair.
If the motor fine then:
Check the pick ups on the keeper plate are not touching the metal chassis bloc.
Likewise after 30 years, where the wire is soldered to the keeper plate OR where it feeds up under the motor, it tends to be squeezed. After 30 years, the squeeze causes the outer plastic to creep round the wire inside, causing it to tough the chassis block and short out.
Check the wheels are isolated on the pickup side (plastic black bush between wheel and axle).
As someone said before, you can fit a china made hornby railroad Jinty chassis. I did this with one of mine, fitted DCC concepts lamps and a smoke generator (after reprinting in early BR black). A temp solution until someone does a decent SR tank loco like the Bachmann Jinty someday...
