FWIW, I have just replaced all eight of my sea cocks. The boat is 35 years old.
I did toy with the idea of rebuilding them, however a mate in the insurance game advised me that if there was an issue of a failure causing damage the insurance company probably would not pay up.
The rational was when a part is that old which you service yourself, and it subsequently fails, the insurance company will probably take the view that you firstly were not qualified to make the assessment of the seaworthiness of the seacock in the first place, and secondly you were not qualified to do the rebuild.
If you do rebuild the original seacocks I personally would let a shipwright do the job, just to cover any potential insurance issues.