The motor was running quite smoothly to start with in fact. I followed the instructions in the service document which can be found in the link on this page. I poured a few drops of oil into the hole on the underside of the motor enclosure while rotating the motor spindle. I can't hear any improvement using a screw-driver as a stethoscope. The motor seems to idle quite well when spun by hand and there's no play in the spindle. The whole innards and motor casing seem very clean too, so there probably isn't much dirt in there. I have oiled the top motor bearing according to those same instructions, by adding a few drops through the holes in the magnetic break wheel. All looks very clean in there.
I have also removed, cleaned and oiled the idler wheel bearings, which look unworn, and run very smoothly. I've thoroughly cleaned the idler wheel, which looks in very good shape. I really can't believe all of this is 49 years old, considering how well it has weathered. It seems to me that these decks really were built to last, and to put up with a lot more punishment than they have actually received. I imagine that being built-in to a clean dry enclosure under a console inside a cupboard has helped too.
More pictures coming soon.