Hi Oscar,
Have a look at the threads regarding starters, if you need field coils, it's probably easier to get a Johnson or Evinrude starter or frame with the 4 field coils and just swap your end caps and armatures into it. The 4 brush conversions are awkward because you have to solder or spot weld the grounds for the coils onto the steel frame.
Also there is good starter info on the Norton Owners Club website, I think they also list the Prestolite P/N for the frame and coils. Even if you have to put new brushes in them it's a much easier job.