How to remove and replace operating unit in a wood casing

I have a unit with the operating handle broken off. How do I remove and replace without damaging the wood casement. The inside of the unit has the cover plate at the handle that looks to be one piece and not removable. The outside operating arms seem to be permanently attached to each other. If I cut the arms off the old unit, remove the two mount screws and remove it from the inside - how do I get the new one in place? I have 5 of these windows. One photo shows the broken unit, one shows a good unit and my casement for all windows. Thanks for any insights!