Take it to a professional aluminium welder, look it up in the yellow pages, I have had mine welded twice, they use special aluminium rods, and I couldn't belive how cheap it was, the welder asked me for 1.50$ but I insisted he should have 3$ to buy himself a beer.
Then again, labor might be a bit more expensive in the states than down here in Paraguay!
It's holding up just fine, mine had a long crack in it, some of the core leaked too but now it didn't loose a drop of water during a four day jungle adventure trip. I crashed and ripped of the radiator shroud which took the radiator mounting hole with it. Luckily I just had to take the piece and the radiator to the welder and he reattached it, no prob. I even bent it back into shape (carefully) and it works a charm. Radiators are way too expensive IMHO.