Bt Doctur

it just seems like the prop was "over seated and was rubbing on the casing or your a victim of severe electrolysys

Something is off , it almost as if the prop was running without a thrust washer to have corrosion like that on the case

in pic 2 it looke like it was attacked by a wiz wheel

Tried numerous times to become a mod but never get any email back from the side mods

Think of it as an improvement to the hulls design by creating the access, Last one I did that way and used teak to cover the incisions. The important thing is to THINK about doing it correctly and not just go in and cut away. Oak, Teak, SS or polished alum will work to cover up the cuts.

Can you surgically make an incision and remove the back deck for access and then do the fiberglass repair? Maybe cut the deck out and repair .Correctly done the repair is invisible.Can you cut and hinge that part of the deck in the way? Post a pic of the deck. Can you even get to the rear mounting bolts ? Thats the trouble with boat builders, they build them as throwaways and give no consideration to repairs or engine replacements

try replacing the impeller. Then drain the manifolds and elbow, Remove the elbows and inspect the block off gasket. If damaged replace using the SS block off plate and 2 regular gaskets

Not knowing what you have makes an answer very difficult

When in doubt fiberglass/glue/ contact cement a block of starboard/ plywood/ plexiglass to the bilge area and mount your screws there.

your hatch should be on a engine wiring side not a house side. I would run a secondary wire direct to the pumps electrical connection and run the other end to a convenient spot to hook up a jumper battery or jumper pack

I think you can tap the holes in the cooler for standard 1/4 pipe fittings

A fuse goes to the ACR side not the main cable , use a fuse rated for your alt output

And if this is a standard type spin on that was installed incorrectly you may have to remove the complete assembly, drain, heat area where rubber O ring is ,then twist off. When installing, OIL the rubber ring AND HAND TIGHTEN ONLY