I have the same bilge pump on an F311 sr1. I found a popped circuit breaker that interfered with the float switch activation. It was on the panel under the folding bench in the back. Good luck as for sure that bilge pump replacement looks like a bear.
The pump comes on when you flip the switch it just makes an ungodly squealing noise. I may put some water in the bilge and see if it loosens up some but more than likely I'll find a way to replace it. A bilge pump is one of those things that you hardly ever truly need but when you do, it better work.
It's not going to be fun though that is for sure. This is one of the few times I'm happy to have the small blocks. I can't even imagine it with big blocks in a 272 engine compartment.