Thanks for all your insights, everyone. When the radiator part broke, I thought it was such a unique problem that I'd better get it up on the boards and start getting suggestions quick. Turns out, I should have checked the past posts first, since this is such a common problem.

While reviewing the boards, it looked like it might be possible to repair the broken part, with an inner copper bushing, some JB Weld, etc. But I don't want to spend the next few months wondering if my repair will suddenly fail. Your comments also let me know that the green (well, green strongly tinged with brown) color of the coolant fluid was a bad sign. I'm getting the feeling that while the previous owner changed the oil regularly, he neglected a lot of other things, like this. So I decided I was probably about due for a new radiator anyway, and ordered one from www.radiator.com. They were very friendly, had a good price ($189 for a Modine radiator), took my call on Sunday, and are delivering it directly to my door on Monday.

Wish me luck with the installation tomorrow!

P.S. Anyone have any suggestions about how to hold the water pump bearing steady while I bolt the fan back onto it? I had a hard time getting the nuts loose with my little crescent wrench, and I'm worried I won't be able to tighten them up enough when I replace it. (This whole project started with a failed waterpump.)