Though the old IT was able to run, it had some severe rust on a couple of the frame members that required cutting out the rust back to good metal and replacing. Though I was not able to get any chromoly tube, the local Home Depot had some mild steel that was the exact diameter and after talking with a couple people I determined that it could be be welded onto the frame with reasonable results. I used .35 flux core made by Lincoln Electric, this is the brand also carried by Home Depot and is an excellent value and quality over most other I have used.
As you can see in the photo, despite not having a good way to really bend the tube to give it a slight curve other than stepping on it propped against a 1x2 piece of trim it actually fit nicely. I'm sure a professional machinist could do much better, though the seat fit back on perfectly.