In these situations, I always use a rectangular plastic tool found in my wallet with an assortment of numbers that allows me to order up a replacement.
FWIW, shipping is a complete crapshoot. You can save a few bucks ordering online, but shippers looooove to ruin radiator boxes. They're all swamped this time of year, too, so I'd order up a new one from a friendly local auto parts purveyor.
I've always found that once you get a leaky radiator out, you'll wonder how it's held together this long, and you'll be happy you're replacing the whole thing.
If you need to limp along a few more days before the replacement or time are available, various plastic repair techniques have been discussed.
I've had "good enough" but temporary success with cleaning thoroughly and using a Devcon epoxy intended for plastics.
Any sot of adhesive or epoxy WILL fail sooner or later. But they can work for a while. Just don't go far from home, keep a close eye on things, and carry a jug of premixed coolant.
Plastic welding can work, but unless you're dealing with some sort of accidental damage on a fairly new vehicle (like leaving a tool lying on top of the radiator and closing the hood on it) it doesn't change the core issue, which is that the plastic is old and brittle. So even if you manage a successful weld, you'll soon have another crack, and/or the seals will start leaking.
