You are on the right path. The plan is the easiesy way to get the problem fixed. Fill the pipe with what ever you can get in it, concrete is the best. I have used fines from the rock crusher. They are cheep will fall into place and make a good plug.

You may want to rework the whole pond some day. That would be the time to remove the pipe and install a new one.

A pond that is thirty years old will have a lot of silt to clean out.
As long as you feel the dam is safe fix it the best and cheepest way you can.