Depending on what is behind that piece, you could clean the back side well, place a small piece of metal (anything but aluminum or magnesium...copper, brass, steel) over the visible crack and simply use silver bearing solder (Stay-Brite is the best by a long shot). Here are the upsides: cost--tremendously less than any welding you can do, heat--much less than welding or brazing (think around 430~440 degrees), strength--greater than brazing (a lap joint is over 10,000 psi). Joins dissimilar metals very well and get this: you can do it all day long with a handheld propane torch. Final benefit? When you're done, dress the joint just as if you had used lead. It sands well enough you may not even need filler and it absolutely won't rust. I've even used this stuff to fill in pitted rust spots. Once you get the hang of it, you may think you've just been given a magic wand. Neutralize the flux with some soda mixed in water.
