Exhaust Valve Stuck Open

Have a stuck exhaust valve on a machine I originally bought for parts but am now going to rebuild if not restore. The valve is stuck in the open position and taking the breather off you can see the lifter move but not touch this valve. Was thinking about trying to soak it with Marvel Mystery oil but not sure if that is the right idea or if something better might be out there. Daughter suggested the infamous BFH, but I am wanting to save that until last, either when I am really, really mad at it or nothing else has worked. The exahust port was open to atmosphere for an extended perios with no muffler attached. So water likely got in Though looking in with a flash light the valve stem does appear shiny and not rusted. It is possible it stuck during when I was trying to get a lack of spark issue sorted out.

It appears the engine might actually run if I can get this issues fixed though I do intend to rebuild it. If I can get it to run I can at least check the other systems (Hyduralic / rear end) for leaks or other issues then drain everything.