Solenoid clicks but motor doesn't run

[Apollos]

I bought this 1993 Marathon in 2016. I know the previous owner and he's one of those guys who takes meticulous care of everything. He had just replaced the batteries and had everything checked out. It's worked great until this morning.

I backed out, moved the FNR to Forward and heard a clunk as I did. This was not the click of the solenoid I'm used to hearing but a definite clunk.

I've read a number of similar issues in the forum and am in the process of cleaning all the terminals. However, I'm not sure that is going to solve the problem.

As I removed the nut from one of the terminals on the motor, I noticed it was very warm. The motor was not hot but the nut was. The cable on this nut goes to the FNR and both ends are stiff and somewhat corroded.

I fear it may be something other than just a wiring issue. Anything which could point me to the problem would be appreciated.

Thanks!