Hello, I am a newbie here...but I have a thought...can you place a new pipe from the backside of the dam, then drop a new pipe from the water side into the drain and fashion a new "top" by welding steel to the old pipe? Something like that? Maybe could use the Helo's for the second stage of the project \:\) Pics may help us to get some ideas of what your working with.

Good Luck!