You can't replace the body of the freewheel. Freewheels and cassettes aren't really meant to be serviced - if there's something wrong, you just replace the whole thing.

Anyway, I see no reason why you wouldn't be able to remove it because one of those prongs is gone. That shouldn't matter. The only thing to worry about is whether that would happen again. Even with two though, I think you'd still be able to make it turn. Also, it's still possible to remove a freewheel without the proper tool (http://www.parktool.com/blog/repair-...uctive-removal).

It's your call, basically. I'm not going to tell you it's a good idea.