Step-by-step..
Good clean ground terminals to battery and frame with strong 12 volts to starter switch.
Bypass the bendix by engaging the push button on the switch by hand. The starter should spin. If not, that switch is defective.
If it does spin, but will not engage with the foot pedal, then the problem is lost motion in the linkage. Usually excessive wear.
My rebuilt 6v starter came complete with a defective imported offshore switch. I dug up a used one out of a bin in the farm shop and got back to work.
