Stove has a built in damper plate above the fire box. (rectangular with a long bar through it)
The damper plate is not warped but the top of the fire box is a bit. There is 3/8 fiberglass that seals the contact when closed. It still seals on three sides but there is a curved gap on the fourth side as seen below from inside the firebox , gap is about 3/8" in the center but disappears towards either corner.
It still performs well but definitely have noticed some loss of efficiency and shorter burn times.
I'm overhauling all the gaskets and trying to figure out what to do about the gap. While the stove has a good design that allows for removal and maintenance of most parts, the warped top is part of the main body of steel so can't be practically removed.
I had two thoughts, one is to just double up the gasketing in the middle of the gap and try to use cement to get the proper curve, but it won't be easy as there really is no gap at the corners as seen in photo so getting the taper right will ideally take different thicknesses of braid. I'm sure I could improve it with not much work, but I'd like to get it tight as possible.
I also thought that a good welder might be able to easily lay a couple of beads on each side of the gasket on the firebox top to make a flat mating surface where it's now warped. I'm not sure how flat someone could make the welds, but if it would seal better than I can do with the fiberglass than i will go that route. The damper plate can be removed to get it out of the way.
I realize the odds of the top not warping further are slim, but I don't think it will much or quickly, I believe the main warpage happened when it was overfired once when I was gone.
Stove may come out to be reblacked so loading it in a truck and driving it to a welder would not be much extra work.