Check Valve /Tubing Kit For 8 and 11 AMA Top-Link Cylinders 05165, Repair Kits, REPLACEMENT TUBING AND CHECK VALVE FOR AMA TOP-LINK CYLINDERSAMA,05165 ,
www.surpluscenter.com
It's most likely metric tubing, the compression fittings on the valve are part of the valve cartridge so not really replaceable, I think there's an adaptor that will convert them to a common size but I haven't looked into it yet.
They make adapters to go from banjo to NPT. You could take everything off the cylinder and attach the hoses directly. You could also take that cylinder to a hydraulic shop, they might have a way to replace just the steel tube that has the leaking fitting.
Hydraulic Top Link Repair Parts - With a wide selection of tiller blades, mower blades, seats, hydraulic top links, and other agricultural implement parts, Agristore USA is the one-stop ...
www.agristoreusa.com
It seems like they are out of a lot of things at this moment, but they are a source for the parts.