If the starter has many miles/hours on it, this would be the time to service/rebuild because later it's more difficult to get out. The top Allen socket bolt is hard to reach.
I believe the Ford Solenoid was added to reduce load on the ignition switch.
Follow the wires from the Ford Solenoid.
Where is the red wire on it going to? In other words, where did the power to it come from?
And does the yellow wire have a female end on it for the male spade connector on the Bosch Solenoid?