Major success. I managed to dismantle, clean, de-rust, and re-assemble a door motor and now it works perfectly.
Beware when taking the motor off the arm assembly, as the lever is spring loaded and does a very good job of slicing your thumb when it springs apart.
My advice, put the arm in a bench vice when undoing the x 7/16" headed bolts.
To dismantle the motor, undo 3 x 1/4" head long bolts, and tap the top off with a chisel and hammer.
To get the brush plate off the commutator, there are two spring clips into the body which need to be released. Once those are released, you can carefully remove the Formica plate holding the two brushes, and just ease the plate over the rotor end, and then the rotor/commutator comes out easily.
I soaked the rusty casing overnight in vinegar, washed in water and dried with compressed air, then carefully cleaned everything, re-assembled the motor, put the arm in the vice, attached the bolts and put the whole assembly back in the door and bingo, it goes up and down quite well.
Now on to the other door ......