This is probably a case of unlocking the gate after the goats have left but there is a product called Q-Bond that comes as an adhesive plus filler powders. I have used this myself and it does pretty much what the PR says it will do in terms of repairing and re-inforcing damaged components. For your old header panel, if you still have the damaged or broken off bits, this stuff ought to be able to bond them back together. With the powder, you can tape a hole shut from one side and then start to close the hole from the other side by alternating thin layers of powder with thin layers of adhesive. The bonder kicks off almost immediately and does generate heat in the process. My next thing to do with this stuff is to repair a couple of holes in the dashboard fascia in my van to restore them back to the correct diameter. Label on the box says Q-bond USA, Paterson NJ, and they have a website for more info.
Nick
Nick