Is it a smart idea to simply use the last slot (photo 9632), put up a complete new wall and call it a day?
You can search "stop logs" and find companies that will custom make them for you to fit your needs. You can show them all the pictures and their engineers could come up with something. I am sure you can retrofit this with something decent.
The dam up north was renovated and they utilized a lot of the original concrete. They had to pour some new stuff and tweak it in and such.
The biggest thing might only be dewatering for the work to be done.
