Genuine tapered plug seacocks are quite easy to rebuild. You disassemble them and clean the tapered plug and the valve body with mineral spirits. Then coat the inside of valve body with using the finest lapping compound (bought at a automotive parts store for hand grinding engine valves.) Then insert the tapered plug and move back and forth over something like 180 degrees while keeping even pressure on the tapered plug in the valve body. Disassemble and carefully remove all of the lapping compound. Then coat with waterproof axle or water pump grease as you reassemble the seacock. It should work fine.
Ball valves and gate valves have ''seats and seals''. Tappered plug type seacocks do not. If they have a seal they either have rubber washers or O-rings at the ends of the shaft. They usually don''t even have those.