I had this problem earlier this season on an Arco starter. I had the solenoid click but no motor action. New 101 starter motor (2 long bolts) from Michigan Motorz and I was happy again. Its a bit of a pain working mostly by feel but the starter is light enough to not wear one's arm out too bad until one of the bolts can be hand-started. I took apart the solenoid on the Arco but didn't find anything obviously wrong, further disassembly would involve cutting something open. My plan still is to check the brushes on the motor and if they look serviceable, get another starter solenoid to ideally have a backup part.
I should mention I took the time to remove both the positive and negative battery cables and wire brush each end and the connection points before going to above. A telling sign was that with the key in the "start" position which should be turning the motor, there was no voltage drop from the battery on a multimeter. Solid 13 volts.
