Hello John, if the top of the stove is warped, it probably was overheated at one time. If the top won't come off easily, you could just turn the stove upside down, clean the area where the warping created a gap and fill with the putty-like furnace cement.
If the gap is really wide, like over 1/4" then you may want to consider getting a piece of door gasket, it is fiberglass I think, not asbestos anymore. Cut a piece to a size that you can stuff into the gap with a putty knife or screwdriver. Remove the piece of gasket, thouroughly saturate the gasket material with furnace cement. I would put he piece of gasket on a surface and using a putty knife, coat the gasket with cement and pushing down on the knife on the gasket force the putty into the fabric of the gasket. Once saturated, stuff into the gap with additional furnace cement.
The reason for the gasket is that the cement can be hard and brittle when dry, and may not fill the gap really well. The hard putty also won't flex if the gap grows and shrinks with heat in the stove.
If you have a good draft, the only issue that the gap presents is added air getting into the stove above the fire. This should not be a problem. However if you try heating on a warm day, with variable winds, and have a poor or reversing draft, you may get some combustion fumes and gasses being pushed out of the firebox into your office.
If you were burning wood, the gap allowing air in above the fire could be an issue, but with coal, it is sort of like a fixed-gap damper, allowing room air up the chimney.
If you have problems, and can send me a digital photo I'll help 'brainstorm' the solution.
Hope this helps. Greg L