How do I combine the left and right hand for a range of measures and make that all into left hand?

So I have Christmas canon but I want to take both the left and right hand in measure 18-21 and turn that into all left hand for measure 30-73 and replaces the chords currently there. That's how the actual Christmas canon is played. Much appreciated if anyone can help me out. It's not as easy as it seems. Thanks.