Hi,
A 1948 Cub came with a magneto, I don't know if you meant it has been changed to a Battery Ignition unit, some Cubs have been changed to them.
The Battery Ignition unit came out in 1950.
Below is a page from the 1949 Cub owner's manual telling how to time the magneto to the engine.
http://farmallcub.com/rudi_cub/www.clea ... age-32.jpg The number 1 cylinder spark plug wire post is originally the top post on the magneto cap.
When removing spark plugs clean any dirt that might be at their bases, to keep the dirt from falling in the engine.
I file or replace the points without removing the magneto or Battery Ignition unit from the engine.
I sit beside the engine on a lawn chair, and use a good light to see in the magneto.
I put a large towel on the floor under the magneto to catch small screws I might drop.
You need a short screwdriver with a sort of wide blade, and usually an 11/32" open end wrench for the nut at the end of the point spring.
If the Cub has a rotary mower under the middle of it, instead of the chair, let the mower down to the floor, and put something on the mower to sit on.
You have to run the engine to let the mower down, so if it doesn't run, you can't let the mower down.
Maybe put blocks under the skids on each side of the mower to make it stable, and then you can put something on it to sit on.
It is not hard to time a Battery Ignition unit to the engine.
Turn the engine to number 1 cylinder TDC, like it explains how to do in paragraph 2 in the link above, use only paragraph 2.
Install the Battery Ignition unit with the rotor pointing to the number 1 spark plug wire post on the cap, which should be the upper right post, looking from the rear forward.
Usually they will run, then set the timing using a timing light.
Below are pics from TM Tractor.
The 1st pic is a magneto.
The 2nd pic is a Battery Ignition unit, there is a car style coil goes on a bracket on top of the unit, it is not there in the pic.
