A new diverter and trim kit would be ideal. The huge whole in the shower wall is oddly shaped and oversized and any trim kit may not cover it well. If your trying to be cheap, take some pliers and confirm that the valve works without the handle, if it does, buy an American Standard single handle trim kit and see if it works, if not return it, may need a universal handle of some type. If the whole is too big in wall for trim plate, silicone a piece of waterproof material over the whole with a center whole just a little bigger than the stem. Hope it helps