Since it is gas burning, if is not already, you can just add a inner
metal vent for a chimney and just use the chimney as decoration. As
for the sinking, you will want to at least stop it where it is at
now. You will need to support it and then get underneath it and pour
the footer to it correctly. Usually 3 foot down and from 8 to 12"
thick. I am assuming the footer for it is insufficient.