Gary,
Having done this job this past season, provided you can remove the seacock from the thru-hull threads, there are several ball valves that fit those standard threads......and for not big bucks........The choice will be to use bronze ball valves wich usually have a stainless ball, or to use the Maralon type which is non metallic...I chose the bronze ones which were cheaper than Maralon..........West, Marine Exchange, or others usually have astocking supply.....