This is a two edge sword:
- Refusing to work on the project will label you as a "chicken"
- Working on the project (as is) will almost certainly label you as someone who can't get the job done
I'd say negotiate the best you can, and try to remove some requirements while making every one signoff to the new scope. If this is not possible, then I'm sorry to say that the company your work for isn't the best company in the world, and you should probably seek your fortune somewhere else.