If it is plastic, I would try some JB Weld. If it is brass, I'd solder it.
It might help to use some mesh like window screen to reinforce the patch.
I just did this exact thing. It's our old van that I plan on selling in a few months, once another family car becomes available. So I figured it was worth a shot at holding it together for a while. It been holding for a few weeks now, so we will see. I expect it will be fine.
The crack was on a flat spot at the top. It's a plastic 'tank' part of the radiator. Some sites suggested taking a nylon tie-wrap, and melting/welding it into the crack with a soldering iron. My repair looked pretty good, but didn't hold up to pressure.
Stories of success with JB_Weld were varied, but since this was a flat spot, I was able to put some heavy window-screen like material on it (some scrap of perforated metal that was in my scrap bin - I'm a pack-rat), and then another flat piece of sheet metal that I cut to fit over that.
Cleaned it all with acetone, scratched out the crack a little in an attempt to get some bonding there, sanded to roughen it up, and acetone clean again. Got a thick layer of the JB-Weld in there, pushed the perf metal into that with the JB squeezing through, layered the metal plate over that. One edge of the metal plate squeezed right under some clips that held the tank to the core, so it had some mechanical backing also. Smoothed the JB around the edges.
A new radiator on-line was maybe $200? And I don't think it is too hard to replace (I replaced one on our older van, and that was simple), but I figured this was worth a shot, will replace it if needed.
I'll disclose the repair if I sell it, may just donate it (I guess you don't get much now, and I'm in a low bracket anyhow). If you plan on keeping it a while, I'd figure that if the plastic cracked in one spot, it's probably weak all over and it will go somewhere else. I just use this for occasional short trips until I sell it, so the risk is OK for me.
-ERD50