There is nothing I know of to repair one.
The seat is probably damaged by corrosion or rust chunks getting caught and damaging the sealing surface.
Since it already has a neoprene line, you might opt for installing a fuel shut off valve in the line instead of replacing the tank valve. The new valves are not the same quality as the original, and twisting on the threaded fitting might start the tank leaking.
An inline valve can be bought at a small engine shop.