A quick update (but no photos at this point): I reinstalled the motor and intermediate gear this morning then hooked up the wires temporarily to test it on DC power. It ran perfectly on the test leads so I placed it on the track and switched that over to DC (from DCC). There was an initial stickiness but after a couple of circuits it ran perfectly, although there was an annoying ticking noise that I traced to the front articulation on the coupling rods touching the spacing washer on the valve gear on each revolution: it doesn't affect the free running so I won't worry about it for now.

Next job is to fire up the soldering iron and hard-wire the TCS M1 decoder I have for it.