Replacing the radiator is the best route, without a doubt, and not all that difficult.
However, to answer the OP's question, if you're a wild and crazy guy who likes to live dangerously and If you're in a
real pinch and
absolutely need to drive your vehicle as is, you could try what I did, patch the tank with fiberglass cloth and two-part epoxy.
I was in a hurry when mine cracked, was leaving town the next day (flying) and couldn't find a radiator locally and needed that vehicle immediately so did what was meant as a temp repair, but then because it worked I wanted to see how long it would last.
I'm still driving that 80 (albeit with the radiator cap on the first click) but only locally and plan to replace the bandaged radiator soon.
The method I used was to get the plastic surface very rough using something like 40 grit sand paper and cleaning all traces of grease/oil/coolant off, then using strips of fiberglass cloth (Lowes sells it) you need to
saturate the strips thorougly working the epoxy between the fibers before you apply it with a two-part epoxy. I initally applied maybe 4-5 layers of that to reinforce the plastic tank maybe four inches to either side of the crack and the full width (top) of the tank front to rear.
KEY part is that the
first layer must be completely soaked/saturated with the epoxy
before you apply it as well as applying some epoxy on the plastic before you lay it down so that first layer sticks well. Then repeat that for at least 5-6 layers. I didn't get that first layer quite right so had to add more layers a few more times afterwards but it eventually sealed off, mostly. You'll want a slow curing epoxy also, the quick curing type won't give you enough time to work.
There is a another ?possible option, use a hot staple plastic welding tool first to put some metal staples across that crack, then melt some plastic and smoosh it across the crack, then use the fiberglass cloth and epoxy over the whole mess. I didn't do that myself due to having no time, but the hot staple tool costs about as much as an aftermarket radiator.
Again, not recommended to patch it unless you just need to do it as a temp fix.
FWIW here's a link to hot staple tool however IDK how well it may work if used on a radiator tank:
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