Is the fireplace on an outside wall?
If so, you could replace with a low level discharge fan assisted appliance
preferable a room sealed one for added safety
I don't know what you mean about a 1/2 height chimney, could be anything really.
And I think most installers would be reluctant to use one by the sounds of it unless
the new appliance specifically gave instructions for that particular flue type
As your appliance is 10 years old, I would assume it has most likely reached it's
useful working life anyway, so a new appliance would always be preferable