I doubt whether you could use hydro-forming here because the header is welded tubular sections. Chances are one of the welds will give up before the dent is pushed out.
However that it made of welded sections is good news. Lian, you'll need to cut along one of the welds and bash it out the old fashioned way and reweld.
Lian, where in the Ukraine are you? I might have a contact for Ti welding (usually difficult to find) for you if you're in the west (Lvov). Of course we would do well to remember that the old USSR was doing things with Ti that the decadent, imperialist capitalist west could only dream about.