Rebuilding heater box, replace core?

I cleaned the damper doors with laquer thinner and steel wool to remove old adhesive. Then 3M adhesive remover with paper cloth towels.
I also removed as much surface rust as possible without scratching the surviving plating.

Remove the styrofoam and the shut off damper can be removed at the top. Just go gentle so the metal flexes and keep stress off the rivets to the fibreglass.