[Graham]

That should clean up ok. It doesn't need to be perfect as long as it can rotate without the corrosion fouling the can. A bit of WD40 on a rag would take most of it off but ideally I would gently lever the tags on the end of the can, remove the endbell and armature so that there is no chance of debris getting in the motor, give the commutator a good clean and check for broken wires on the armature poles.
John Secchi told me you can unwind a broken wire one turn and solder it back on with no effect on the motor or take a turn off every pole and you will gain a bit more revs.

Hi Tony,
my message box was full, so i have cleared some out and sent you new messages