You will need the missing piece. If not replaced, there is nothing to properly hold the spring in place. It looks like the present set-up used a nut to achieve the same purpose but I would need to see where the brake mounts to make a better call. If it was working, so be it, but brakes are pretty important on a bicycle. You should try to get it right. Chances are you will be able to find another, if you look around.