If it is a 1951 Cub, the wiring is different than it shows in the 1949 Cub owner's manual that I posted on page 1 of this post.
In 1950 IH changed to the voltage regulator on Cubs, and the Battery Ignition unit, but a magneto was available also, it could have either one.
The voltage reg is mounted on a bracket next to the generator, not on top of it like the Relay was.
Below is the 1955 Cub owner's manual, they had the same charging system as a 1951 Cub had.
It also shows the battery Ignition unit. You should check the points and other ignition parts, the points get old and burned from use. Cubs start and run better with new smooth points.
It shows the Battery Ignition unit points on page 41.
http://farmallcub.com/rudi_cub/www.clea ... index.html
TM Tractor has new points, so does NAPA, they are CS 1600 at NAPA.
The listing show you what they look like.

http://www.tmtractor.com/new/el/367fp.htm