I tried to get a quote on welding a patch on from Atlantic Spars today and they said they never repair fuel tanks. They would gladly build me one for about $1,100. John Justin gave me a ballpark estimate of $450.
I examined the inside of my tank today by putting a digital camera into the access port and taking photos. In general, the tank looks to be in quite good shape inside. The welds look somewhat like they are rusting, which they're obviously not, but that may be how an unpolished weld in aluminum looks. I am really tempted to try the epoxy and fiberglass method that the West System folks suggested so I can get the project done and go back to doing some sailing.