sounds like those valves are properly seized!!!
you do not, under any circustances want to loose the relative position of the valve sleeve (tube). knock it back in and stamp the end of the sleeve with a corresponding point on the frame casting.
you then have 2 main options.......
drift the whole lot out and soak it in parrafin or similar, then unseize it, then reassemble using your marks for alignment,
or I would get 4 really heavy washers and use the tapped holes for the valve cover, with the washers to prevent the tube walking out. then drift the valve out as previous.
edit.
I would guess the valve sleeve is 1 piece.
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