You need to pull the starter and have it bench tested and while it's off see if you can turn the motor over. Seeing the pic now it looks to me like it's been wet for quite a while and the insides are probably looking the same as the outside.
The bottom terminal going into the rubber plug is the starters main power. Without that you have no ground to most solinoids. Must hook that back up to the solinoid.Hot on the biggest pole shorted to the S will cause it to crank over.