That appears to be equal parts rust and moist fuel residue with a healthy deposit of evaporated fuel goo at the bottom. It will take a bit of work, but is savable if you are willing.
Acetone will dissolve the fuel deposits, and acid will remove the rust. Unfortunately, both will also damage the paint, so some care will need to be taken.
Remove the gas cap. It's just going to get in the way, and you will disassemble and restore it separately.
I'd begin by putting a straight level across the cap, and putting the tank on a very secure work top where it can remain undisturbed for long periods, and leveling it. It is fortunate that the holes are near the very top. Tape over the holes with non-vinyl and non-plastic tape such as painter's tape. Fill the tank to the very brim with acetone and monitor. It might take hours for the goo at the bottom to dissolve. Keep an eye on the taped-over holes.
You will need a strategy to remove the acetone without getting it all over the tank paint. This is what I do. I blank off one of the petcock holes, and I put a simple drain on the other petcock hole. The little wooden dowel on the left-hand drain slides out easily, allowing me to open the drain without disturbing the full-to-the-brim tank. You need to drain into glass containers or the original metal containers. The chunky stuff will settle to the bottom of the containers, and the acetone can be reused indefinitely until it eventually all evaporates with use. Repeat if you think it will help.
Now you can repeat the whole process with acid. In my initial post I suggested vinegar, which will work, but might take days. Muriatic acid from Home Depot will work faster, but still might take days. When satisfied, drain, rinse, and neutralize with tap water with a little Clorox or baking soda in it.
This is a basic guide and strategy, but you will have to pay attention to what is happening and adjust accordingly. The end result does not have to be spotless. Even if it is spotless, it will immediately flash rust when it dries. This is nothing, you can apply Caswell's over the rust, you just don't want the rust to be thick or loose.
One more thing. Despite your best efforts, you will inevitably get acetone and acid on the paint, so you should take measures to mask the tank with clear stretch wrap, or even garbage bags, or aluminum foil, or both.
If it sounds like too much, I'll buy the tank.