Do you have the seatpost that goes with the frame? I'm not quite sure what that black band is on the linked photos. Tape? Does it use a quill/internal expander to hold it in place? That could be one part that would be really difficult to find if you don't have it.

It is hard to tell if the seat tube has been damaged near the top.

I still think one could make a ring to go around the outside of the tube, maybe using heat to help form it, then brazing it into place. You'll lose some of the original clean lines, but it might not look too bad if done well.

Replacing the tube wouldn't be for the faint hearted, but would theoretically be possible.

I'm seeing a couple of companies that make oval and teardrop chromoly (4130) tubing.
Streamline and Oval Tubing Suppliers - ProFormance Metals
Unfortunately, I think it is all straight tubing (.035 or .049).

However, if they do custom fabrication, they might be able to shape commercially available round bicycle tubing. Or, if your tubes are a true oval and not a teardrop, I wonder if they could be flattened between two steel bars/plates, or even wood boards using clamps or vises.

As others have mentioned, the true commercial cost of the repairs will be very high depending on how you choose to do it, and you still end up with a repaired/repainted frame that some people will shun. But, if you do it yourself, or find a friend of a friend to do it... perhaps it could be done at a reasonable cost.

I would suggest asking to get this discussion moved here. Don't start a new topic, just click on the "Report" button.
Framebuilders - Bike Forums