Good radiator shops can replace the top with a new one. Cost should be less than $100 total. It takes an expensive machine to loosen and retighten the 'crimps' holding the top on to the core, and only radiator repair shops can afford them.

One CAN patch the crack using fiber-glass just like patching sheet metal. Roughen up the surface with 100 grit sandpaper to give the patch something to bite. May buy you years of service. May fail in a few weeks. Just don't overdo the hardener or it will be too brittle.