it goes backwards, but you need to undo the threaded section from the front with a very long screwdriver first, which also release the pressure on an o-ring that secures it (and seals the valve). Two additional small screws secure it from underneath, plus obviously the trans port holds it from sliding. It's a compelte strip down, but not really very hard, assuming you have the right tools, including the ring spanners for the end.
PS very much doubt it needs a new valve; possibly a new valve stem, or the old stem might need a little smoothing of it's sealing face with emery if it's leaking.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.