Generally speaking it is not a standard practice to repair craks in plastic
radiator tanks. They can be replaced though. If the radiator core is good
and only the tank is cracked, the standard procedure would be to just
replace the tank. However, in many cases the cost to have a tank replaced is
about the same as purchasing a whole new radiator.

There are those who have claimed to have success at repairing some cracks in
plastic radiator tanks, and there are various methods that have been tried
without success. I have never personally seen a patch that held for very
long, and I know that just using some J B Weld on the crack will not usually
work. You could try a Google search to find links to various remedies that
you might try.

It's certainly worth a try. What do you have to lose? My daughter's Mazda
Protege had a crack in the top of the plastic rad at the base of the pin
that holds the rad in place. It was leaking a lot more than a drop every
few minutes. I scuffed it up around the crack, cleaned it real well with
brake cleaner, and applied a glob of JB Weld, working it into the crack
as best I could. The rad was warm and I let it cure for a few hours
before topping it off & giving it a try. It held fine for over a year. I
finally replaced the rad when I did the timing belt, water pump, etc. It
mighta held forever but I didn't want to risk her being stranded
somewhere. If it were my car I woulda left it. I was surprised that it
worked so well. As always, YMMV...

Don't JB Weld, epoxy, or super glue it.  The best repair is done by
melting a groove where the crack is with a soldering iron (use a
completely clean tip or cover it with aluminum foil) and filling the
groove with more of the same plastic.  Radiator tanks are almost always
made of fiberglass reinforced nylon, and rods of this material are
available from GM dealerships or radiator supply houses.  Don't use the
wrong type of nylon, such as from a plastic wire tie.  Nylon gives very
strong repairs when melted together.