Based on a comment from a fellow at a radiator shop, you are out of
luck.

Plastic radiators cannot be repaired the way you are hoping.

He said if it breaks, buy a new radiator, it isn't repairable.

There's nothing you can do that will be reliable, replace the radiator,
unless you would rather spend $5K on an engine rebuild when your fix breaks
on the highway.

Plastic radiator tanks are STUPID STUPID STUPID, I'd like to smack the
engineer who came up with those. They're a time bomb, they *always* fail, I
have yet to ever have a metal tank radiator fail catastrophically like the
plastic ones do.

Replacement is as others noted the only way to fix a break anywhere on
the plastic tank.   It is a horrible design that was likely done to
shave a few ounces off the weight and several dollars off the cost.

Has anyone found a good metal tank replacement?

Ok, radiator is shot so why not experiment?
I would think along the line of a metal hose fitting ( same diameter
as the connected hose) that you could drill a larger hole in the
radiator top, in the same spot as where the nozzle was...... get a
threaded fitting and place it in the opening using a neoprene washer,
metal washer and a hex nut and tighten to avoid leaks. You might have
to find a way to get the washer/nut inside the radiator housing.
Try your plumber's supply shop or an industrial fittings provider..try
Parker Haniffin Mfg. (Tell them you want to add a drain hose to a
metal can and they will come up with such a device.........i guess ;)

BTW some radiator shops will replace the plastic headers if the core
is good.
Had it done on my 1998 760 for $70.00

E mail me a picture of the situation i might be of help,
<proaire@gmail.com.>