Easy fix! Unscrew the bar copper wire that is attached to the box and attach your new green fixture wire to this with a wire nut. If the old bare wire is really short, extend it by adding another length of bare copper wire or green #14 gauge wire to it using a wire nut and then proceed as instructed above. When you attach the new fixture to the old metal box, the mounting screws of the new fixture will act as the ground for the box (which was the purpose for the old bare wire being wrapped around the screw of the box in the first place).
Hope this helps.
Phil
Hope this helps.
Phil