A picture of the damage to the header would be good just to see what specifically needs to be done with it. But being a body guy I would start by, grooving out the crack with a cut off wheel or grinder, then grind the front and back sections of the area to be repaired. then apply SMC/fiberglass repair material to both sides with a layer of fiberglass mesh glued onto the back side of the area to be repaired with SMC. That will give the part its strength back. Then when the glue is dry you proceed with normal body work by sanding the glue down and using regular body filler/poly putty to smooth the area out. Follow up with primer and paint and your done.