Maybe I'm oversimplifying but since it sounds like your planning to drain it: Could you just install the siphon, use it to lower the pond, remove 10' or so of the damaged overflow pipe and repack the trench with clay and also remove the silting while it's drained and equipment (I'm thinking track hoe) is on site?



"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge." Stephen W. Hawking