Repairs Honda 20hp MOD: GXV620K1, ENGINE: QYF

I have a client with a Honda 20hp MOD: GXV620K1, ENGINE: QYF. It has an intermittent starting issue. When the ignition key is switched on, sometimes it will fire right up. Other times it clicks in the starter. The clicking is caused by the starter gear plunging up against the flywheel but not rotating. I started by testing the system at the battery = good. Next I moved on to the safety interlock system of the mower (Hustler Fastrak) just to rule that out and that passed. Next I tested the voltage coming from the positive battery to the solenoid = 12.84v. I then tested the voltage coming thru the ignition switch to the solenoid spade = 12.84v. So I replaced the piggy-back solenoid with a new one. Same problem. So then I checked the valve lash and found that to be fine, within specs. Finally, I moved on to the starter and replaced it with a new aftermarket starter from Oregon Parts. The starter will receive electricity and plunge the gear but won't rotate. Once again I tested all voltage going to the starter, and I am getting over 12 v. Do you think the new starter is bad, or am I missing something? What is the proper way to bench test these starters? For starters, I have always just applied voltage and ground and either they plunge and spin or they don't. But I don't have much experience with this machine. What else could this issue be? Thank you to all in advance for your time and experience!