I unfortunately just went thru this with my fresh water intake for the engine. It really isn't as hard as it may sound to replace the whole thing. I started backing off the old gate valve and it just kept turning. Come to find out the original "thru hull" didn't have a flange on the outside and was really nothing more than a piece of bronze pipe glassed into the hull. I ended up spinning out the whole thing. ground down the inside of the hull to match the outside contour. (was 4 in. thick) Used a half round file to enlarge the hole to the desired size. Installed new thru hull with lots of 5200. After that set up I then put the ball valve on and reattached the plumbing.
Whole thing took maybe a half day total time. Did it the week prior to the boat going in the water. No problems.
hunter Legend 37 Semper Paratus
Formerly - Tartan 34C Yawl