Finished cleaning innards with wet and dry and petrol. Brushes and commutator were all in good condition, but now at least cleaner. Drive pinion turns smoothly. Shafts regreased and whole thing reassembled. Tested by hooking directly to known good battery – works fine. Seems to have been a pointless excercise, other than making it shiny. Tomorrow: put starter motor back in and test.
Picture would have been nice… but I forgot.