To install, they run a stainless steel liner up the chimney. The insert and the liner don't really depend on the integrity of the old fireplace at all. Running the gas line is sometimes the trickiest part.

with blowers - on Craigslist for similar reasons. It's fabulous. Very hard to move, we built a trolley to install it.

At that time (and it may be different now) a fireplace insert was essentially a specific type of wood stove which fit into the firebox of a fireplace and vented up the chimney. As a wood stove they were designed to allow the wood to burn more slowly than in a fireplace, and as a result, required cleaning at least once a year if not 2-3 times, depending on frequency of use.

At the time, I charged $45.00 to clear a fireplace/chimney, $75.00 to clean a wood stove/chimney, and $150.00 to clean a fireplace insert. The price differential was because I had to pull the insert from the firebox to be able to clean the chimney. Pulling a 300-450 pound cast iron insert by myself was no easy job, and required me to bring in concrete blocks and a heavy plywood sheet to built a platform for the insert to sit on while I worked. In short, they worked well, but I hated them.

Some quick research also indicates that an insert isn't a solution for your problem, at least with respect to a specific manufacturer of precast fireplaces.

http://swedesweep.com/rampart-chimneys/.

Yup, that is the critter. I'll check with the city but I'll bet the only way to deal with this thing is to have it replaced. Sad face emoji dollar sign.