Thank you all so much for all the good answers. Sadly, we have reached the point where anything too much more time-consuming will be a time-sink. Talked with store owner and they want to just rebuild and re-enter everything.

However, I very much appreciate the process -- I've learned quite a bit -- and also will be challenging myself to learn a LOT more about databases and stores!

PS And the most important lesson I've learned is to advise client to upgrade every version -- not wait 7 years!