The case screws are #3 JIS. Fortunately a hand impact driver with a #3 SAE phillips bit will work fine without further damage to the slots. The JIS screwdrivers are useful but unless you have one of the Vessel impact screwdrivers, a regular hand impact driver works a bit better. It also helps to put the screwdriver in the slots and tap it sharply as this will cause the screwdriver to seat a bit better and help break loose the stubborn case screw. Remove the screws and make a template out of cardboard with an outline of the case cover and then put the screws holes in the template to keep track of which goes where. I suggest you invest in some socket head metric bolts as these are less prone to damage and look a lot better than the originals with the worn slots.
Texas Hill Country