First thing you need to do is stop propagation of the crack by drilling a relief hole at the end of the crack. You can even do it with a pin, filed flat at the tip, in a variable speed drill, going as slow as you can, don't put pressure on the disc, let the drill bit do the work. Put something like Saran wrap over the data side, then put the disc on a rigid flat surface, like a plywood scrap, then drill. You don't want any flex in the disc while drilling or the crack will grow. Once you have the hole drilled, use a drop of super glue in the hole and one at the other end of the crack. Use a new razor blade to trim anything sticking up above the disc surface with radial scrapes. Be careful not to put scratches into the data area in any direction other than straight out from the center(radially). Scratches can be removed with a few quick passes of a pencil-type butane torch. This flash-melts the surface, which becomes flat again, filling in the scratches. It's a technique you want to practice on old CD's first.