Find thin walled copper pipe, CPVC, or a steel nipple that fits inside the broken neck. Copper pipe or CPVC works well because you can get an oversized one and cut a slot so that you can compress it and it will spring back tight against the ID of the neck. Clean and rough up the surfaces on the inside of the neck and broken piece and outside of copper/CPVC, slather one end of nipple with JB weld, assemble nipple into the broken piece, slather other end of nipple with JB Weld, then insert into the radiator neck (index the broken piece to the radiator for a good original fit), lightly clamp together, clean the squeeze-out, and apply heat with a lamp. Wait at least 24 hours.

A hillbilly fix that has worked for me.

It might be a good idea to double clamp the hose, depending on where it broke.