If You do not have the nipple that came out of the neck, go to a recycler, most of those was all the same diameter, take along a torch and remove one from a old radiator, I am sure a recycler would let You have that for free.
Take that home and warm it up, wire bruch the crap off of it, then, use the torch and some solder and "Tin" the outside where it will fit into the radiator.
Next, take a wet rag and lay it around the neck of the radiator, this will help to dissipate unwanted heat and loosening the neck joint, wire brush and sand the neck surface real clean, apply some solder flux paste with a little heat and tin the area around the hole, insert the nipple and heat it enough to get the solder to flow, let cool and finished.
Real easy once You get everything properly tinned.