One window or several? Many of the older windows were shaped with a customized hand plane. Might be easier to have a blade cut to the shape, then do the trim by hand. I wonder if Hock or some metal shop could duplicate that curve. If you had such a plane, you'd own all future repairs on othere windows in the structure. Alternately, a roundover type bit would get you close, then you could use a small plane to shave it to a flatter curve.