As you say, the gaskets are not that difficult. Many are adapting the mercury float, as in the illustration. I had a collection of punches and punched out needle valve packing out of the neoprene impregnated cork I used to make fuel tank gaskets. The cut out in the center of the fuel tank gasket would make dozens. a stack of three or so would keep the valve from leaking and hold it in place. Do not allow the packing nut to bottom out. over tightening will break out a part of the threaded portion and render the casting useless.
