Ifishnow,
If the existing pipe and/or riser is rusted out the most usual option is digging both out and replacing with a pvc pipe.
Plugging off the existing pipe and installing a siphon system might also be an option. (haven't tried that).
Digging through the dam and rebuilding it is pretty safe.
I do not like the idea of cutting a drain in the spillway. Spillways are placed on virgin soil when designed, never fill. If a flood happened, where the fill was at in the spillway could erode a lot. Water shouldn't go over the top of the dam so fill in the dam is safer from erosion from water. z
