is mostly metric. You might be able to get an imperial seacock flown over from a US chandler or you might replace all parts
up to the hose. If it's 1/2 inch, you should be able to get a seal on 12 mm nut and tail even though it's slightly smaller. Most imperial hoses have a similar sized metric equivalent. You might try a UK chandler as they might stock imperial parts
for old boats. In Aus we have been metric for over 40 years but they still sell imperial nuts and bolts in hardware
stores.Timber is sold in multiples of 300mm because that's close to the old foot measurement.