Window header replacement in stone facade

We have 2 windows with damaged headers and need to replace the header and windows. The windows are in our stone façade. My question is how much stone needs to be removed in order to repair/replace the headers and install new construction windows (not replacement). If headers don’t need repair (as in 2 other windows) in same façade, how much stone needs to be removed to install new construction windows? We want to minimize how much stone is removed. We have had dry wall removed from inside the house to check the plywood for any damage. Everything checks out dry with moisture meters so we are fairly confident the 2 headers are the only damage.