broken exhaust manifold bolt

Last night I discovered a broken exhaust manifold bolt on the passenger side of my 89 s-10 blazer 2.8 . I have noticed a slight use of water, but none in the oil. Today, I was driving to the store and my temp started rising. I pulled over, and to my surprise water was pouring out of the manifold area where the broken bolt was. I guess my question is...after 189000 mi. should I replace the whole engine, or can I get away with just replacing the bad head with a used one. I need to drive back and forth to school and to the store. thanks, mike