I'm not sure if the valve is included in the link below, but I'd measure what you've got and then replace the entire thing as they're only about $40-$60 for the assembly, and then the bracket is an extra $15. Though, it looks like something you could make quite easily and cheaply if you had the materials and tools to do so. If the valve isn't included, you can get those for about the same price of $40-$50, so you should be able to do the entire thing for about $100 total, again depending on what size you need.
Makes operating hard to reach seacock valves much easier. Operate seacock from the deck Stainless Steel construction Mounting brackets available to keep arm in easy to reach position Custom lengths available for boat manufacturers Brackets not included, sold separately. OEM/Bulk Product Name...
thmarinesupplies.com
Mounting brackets available to keep arm in easy to reach position OEM/Bulk Product Name Distributor Packaged SCB-527 Seacock Arm Mounting Bracket – 4” Length SCB-527-DP
thmarinesupplies.com