Most engines have replacable valve guides, removing and installing is ussually a specialist job requiring the proper tools or you could ruin your engine. They are pressed in and need to be pressed out, not hamered.
First thing i noticed though is the state of the valve seat, looks pretty damaged on the bottom of the picture. You are never going to get a good seal between seat and valve there, grinding in a new valve will not be enough.
They need to be machined and you probably need hardened valve seats installed to repair it. There are valve cutting tools that use the valve guide as centerpoint to "freshen" up the seats. But again this is better left to a specialist since most of the time the seats need to be cut at different angles. Again getting it wrong is ruining your engine.
As for cleaning the engine for painting, use paintstripper, steel wire brush degrease and spraypaint with heat resistant paint. However the best way is to plug any holes fabricate a cover for the top side and sandblast it with a rough grain, this cleans it up in the best way possible and leaves a clean but rough finish for the new paint to stick to.
