You can look at Steve's thread
http://www.quiltingboard.com/vintage...e-t241418.html He's currently repainting this WW 9W using modern paints.
I think it was Glenn (if I'm wrong forgive me..) that mentioned using shellac mixed with black pigment to repair small areas of original finish. That would be a sort of homemade japanning.
My thoughts are I wouldn't repaint an original machine with a good finish but if the finish is shot you're not going to do any harm. Newer, common machines are fair game too.
Rodney