Hi,
See if you can get the pipe size that connects to the value and then match it up at a good marine hardware store. The biggest thing is to get a quality marine grade ball valve made from bronze or DZR brass (look for a CR marking on the seacock). CW617N marked valves have high zinc - ok for domestic water or fresh water environments; not salt water.
You could always check with GW Customer Service via the net and see if they have information.