Hi Sean,
Assuming that the bronze hasn't turned pink deep into the metal, these Perko seacocks are rebuildable. The parts needed are:
080500599B Perko Repair kit - 3/4
080500599C Perko Ball Valve Stem (driver)
which are available by special order from Jamestown Distributors, Yacht Supply Depot and others.
I had good luck rebuilding mine, and found that the replacement parts were better than the original.
-Chris