I have an older Fox-Sterlingworth shotgun that was stored, in a soft case, in an area below a water leak in our house. Apparently the slow leal had been going on for some time unknow to us. Anyhow, among other problems, the shotgun has a great deal of rust on the barrel and receiver. Any suggestions on how to attempt repair of this disaster?

First thing you have do is "Stop the Bleeding", or, rusting in this case. You need to completely strip the entire gun and give it a thorough cleaning and then try to determine the full extent of damage. Here is where you need an experienced eyeball looking at the actual damage. Take it to a reputable gunsmith for his evaluation. I've saved some pretty heavily rust damaged stuff in my time, so anything is possible. Not always practical though.