If it is just a stress crack where the top tube goes into the top (or bottom) of the radiator is can easily be torch soldered.
Take the rad out, lay it on it's face and wire brush the half circle on the cracked side. Leaving paint and scum on the good side helps prevent the solder on that side from flowing.
You only want to do one side usually because if you get the whole shebang too hot it may want to just fall out!
If you want to solder the whole circle, you need to prop up or wire up the tube as it might just fall out when you heat the whole thing.
Anyway, clean it up good and smear on some plumber's paste flux.
Next, strip the insulation off some stranded copper wire (12 or 14 AWG) will do.
Untwist the length a little to loosen the strands then cut a piece that will cover the leaky seam and then some then lay it down in the fluxed seam.
Have some water and fat plumbers acid core solder handy and fire up the MAPP torch.
As you are getting it hot, it helps to touch an area near the seam with the solder just to read the temp and know when it's time to apply the solder.
You want to get the one side of the tube hot enough to sink a couple of inches of the fat solder wire all in one shot.
The more times you re-heat to add more solder, the less chance of success you have.
The stranded copper wire filet greatly adds strength the to seam.
Just as soon as you sink in your solder and get a good flow, douce the tube with water and you are done.
Again, just try not to use so much heat that the tube completly unsolders.
You can even put a small piece of wet rag on the other side to prevent the original solder from flowing.
Since you've done a lot of plumbing, you get the idea, It's just like soldering copper water lines except the rad tube doesn't have a sleeve fit and not a lot of joint contact so you have to be careful to keep the tube in place during soldering. You have to consentrate the heat and do a quick and complete job of it before you have a chance to unsolder the core.
Done correctly, you new joint will be stronger than original due to the copper filet. This also works well on the notorious lower tube crack that you get from hoarsing off and on the hose.