Remi,
I dont throw around the term "have a pro look at it" alot but in this
case your desciption of "a lot of missing mortar" conserns me. If it
is that way in the hearth it is bound to be that way in the chimney
and you need to know if it is. Cracked or missing mortar is a seriouse
problem and can easily lead to a house fire.

If that suggestion doesnt appeal to you wallet but you still like the
fireplace then you are just like me. My fireplace was in bad shape due
to mortar issues, a bad crusty worn & damper, but I still liked the
look of the fireplace. I didnt want to bother with repairs so I bought
a nice candelabra and a chimney balloon. I use the candelabra and
light it up when I want some abmbiance (just like I used the
fireplace. Doing this actually saved money on my gas bill since the
fireplace didnt suck the heat out on my home when i was using it. The
Chimney Balloon helped me seal up the damper nice and tight when i
wasnt using it at all.whick also prevented the den from feeling so
cold in the winter. I know this is a diferent way to look at the issue
with your fireplace, but the best I can offer is waht worked for me.