You could try popping down to your nearest custom exhaust builder.
They will have a hydraulic pipe expander designed for the job.
Failing that, it's possible to buy internal exhaust pipe expanders, which work on threaded rod. I've only ever used them for making a clamped, distorted pipe round again, so not sure if one would be up to the job.
Have a look at the adverts for one, then it should be easy enough to duplicate in the workshop.
This sort of thing on youtube; https://www.youtube.com/watch?v=zcz6CSVmxQU
N.B. turn the sound down, and scroll to about 1 Min 45 
Bill
Edited By peak4 on 16/10/2017 23:49:00