find someone with a internal hole gauge to try to figure out what the right seatpost size is and get one that size. It does make sense that the undersized seat post caused the problem. Unfortunately, with a crack that big it might not stop growing. In fact, my thought is that it ends at the white dot that is almost all the way to the weld. I think you could braze half of a tube as an overlay on top of this, but it would be hard to make it look right.