I used to do hvac as an assistant. There's lots of thought that goes into an install.

What's your chimney liner like? Do you have a chimney liner? Is the interior of your chimney collapsed? Can you accommodate a direct vent setup? Do you have electricity at the fireplace? Is that gas line even still connected?

The previous owners removed the insert for a reason. Finding out why is a good step.

This is why you call a gas tech. They can come by and for a small fee give you an idea of whether or not your setup is workable, or if it's not. You strike me as someone who likes to save money and if you've gotta replace the chimney, that's not cheap and not to be done by amateurs. Further, the majority of insurance companies will fuck you sideways if you so much as breathe on a gas appliance without being licensed.