If you could somehow get the remainder of the seat post out, you could probably put a new one in and beat the dents back in with a wooden punch.
The NDS seat stay cap looks funny, hopefully it's still attached.
If you do replace the tube, just get a modern tube. You can find NOS sets of 531, but it doesn't seem like a good investment. To replace the tube, cut a 3" section out of the middle of the tube. Optionally thin the tubing left in the lug. Heat the tubing from the inside, remove. Repeat on the other end
This is why I don't recommend mechanical destruction of stuck seat posts. Did you try heat?
Last edited by unterhausen; 01-06-11 at 10:39 PM.