Really, about all you would need to do is put the 12V batter in place and change to 12 volt light bulbs (after you decide what generator or Alternator you will use).
That decision will determine if the system will remain positive ground or neg. ground.
12V is NOT necessary on a JD "B" and if you do want 12V I would recommend reducing the battery and ground cable size to reduce the amperage available to the starter. 12V will spin that poor starter super fast..