Try pushing the switch button on the starter without engaging the pinion gear by pushing the stomp pedal. The starter motor should spin without turning the engine. It's possible you've got bent or damaged linkage that's not moving far enough to push the button as the gear engages the flywheel. Ditto on bench testing the starter before it's installed. You could also try gluing a flat washer onto the switch button with RTV sealer or trim cement as a quick test of the linkage to see if that makes the motor turn as the stomp pedal is pushed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!