I think they usually recommend a small engine plug but I just pulled auto ones out of my 6hp kicker... So they must work. Im actually changing mine tonight and getting my plugs from shucks. Search online the the year and brand of the Johnson and you should find all the info.
Different motors take different plugs. look online and you can find the right ones or go to a boat shop and buy them. I had the wrong ones in my evinrude and it wasn't running right so i took it to the shop and they pulled a plug and said whoops we put the wrong ones in and switched them out and it ran fine afterwards.