Converting old stove chimney to standard fireplace

Hi all,

We have just removed our old kitchen and want to convert the space to an office/study. We would like to install one of those mantels/cast iron inserts/small (decorative - more looks than heat) gas burners however the opening for the original stove is higher than that of a standard fireplace.

What are our options to fill in the top 500mm of the opening to be able to install a standard mantel? Does it have to be bricked in (I'm reluctant to start removing bricks to key in new courses - the old mortar just crumbles) or can I board it up in some way to look like the rest of the wall?

Any suggestion greatfully recieved.