Pop the top cover off by removing the 4 screw from below. I use jumper cables to put 12 volts to the 2 big wires, one red one black running down the tube to the power head. If the motor does not spin then you have brush or armature issues. If the motor spins you have board issues since your controller worked on someone elses motor.
It's been a while since I was in one of those, but they have a relay board that craps out, it's in the base and not hard to change or extremely expensive.
The clicking suggests the relays are closing, so, with the foot switch in the constant position and speed in the middle somewhere, probe those two red/black wires for 12 volts first. If they are getting 12 volts the board is OK and you have brush or armature issues. I would probe those wires first, and do the jumper cable thing as a second confirmation test.
The armature and or brushes are not terribly hard to service, you just have to be careful to get everything back where it belongs.
Look to the parts sellers websites for diagrams that show the insides of the power head. My current V2 powerdrive has a cardboard ring that holds the brushes back for reassembly. You put the ring between the brushes, then slide the armature and housing in to the base housing. The armature pushes the ring back into the housing and the brushes pop on to the armature.