When we purchased our boat the surveyor missed a few items, one of them was a broken seacock. An obvious piece of advice when buying a boat is to open and close every seacock. The seacock in question was the head discharge, the largest one on the boat.
I am a big fan of the Compass Marine website, this link describes their recommendations for replacing a seacock with no through bolts.
To remove the old seacock you need a seacock wrench.
With some effort the flange will spin off.
Here is the old seacock removed.
It seems all new seacocks are triangular while all the seacocks on Mason sailboats are round. This means that the old backing plate had to come off.
The backing plates are glued on and come off pretty easily with a couple of taps with a hammer and a chisel.
When making a new backing plate it helps to have a friend who was once a shop teacher and has built their own home workshop. Charlie Michels made quick work of making a new plate ( he also has a great selection of Dogfish Head beer).
Comparison of old and new seacocks and backing plates
The hull was curved enough where the seacock needed to be seated that I had to go back and sand down the edges to get a snug fit.
Using flange to align backing plate while fiberglassing in (through hull wrench needed again)
New seacock in place
A healthy amount of BoatLIFE Life-Calk was used both around the flange as it was tightened and to seal the seacock to the new backing plate.