Spartan Seacocks are a nearly "life of the vessel" product. They can last nearly indefinitely IF PROPERLY MAINTAINED.. I would strongly advise against replacing these seacocks. You have some of the best & longest lasting seacocks ever built...
They likely just need a clean and grease or at worst a clean, lap fit and grease.
The PROPER GREASE needs to be used. People often try to save a penny or two and opt for a grease other than the specially formulated Spartan Grease. This grease is the consistency of cold peanut butter and works better than any other grease for this application.
A plug inserted from outside the vessel to stem the flow can be used while you re-grease the cone. I would not "lap-fit" in the water as it can be tough to flush all the valve grinding compound out of the valve once you are done. No valve grinding compound can remain anywhere on the valve or cone after lap-fitting.
DO NOT over-tighten after greasing. People always tend to make this mistake. They only need to be tight enough so the handle does not flop closed under its own weight.
If you service these yearly with the correct products they can easily last well beyond 40+ years of service. I have plenty of customers with tapered cone seacocks well beyond 40 years and they are still working like new....
These are the "officially sanctioned" on-line instructions for servicing Spartan tapered cone seacocks.. Servicing Tapered Cone Seacocks
If you service these annually or at least bi-annually they will give you a very, very long life...
Call Paul at Spartan Bronze and order some grease, a spanner wrench and some valve grinding compound, then enjoy your seacocks for another 30+ years....
Oh, he can also sell you a new handle..