Insert help and chimney liner

[T_BOLT]

I have a house built in 1987 and bought it in 1989 it came with a fireplace insert. We use the insert approx 10-12 times a year. The chimney has never been cleaned. I planned on having it cleaned this year. In doing research I learned that I should have cleaned it a lot sooner. But I now have more questions. I have learned that the chimney does not have a liner, so I was planning on installing a DIY chimney liner.

Now my problem is when I was looking for the right liner kit I needed to get a rectangular exhaust opening boot. When I took the face plate off to measure the rectangular exhaust opening I noticed that the top of the insert has bowed down around the exhaust opening.
How serious is this? Could I possibly use a jack and 2x4 and bend the bowed section back level?

I dont want to proceed with the chimney liner if the insert is useless or unusable. I do not have the funds to replace the insert and do the chimney liner.
I have attached picture to better show what I mean about the bow on the top of the insert.

What are my option at this point?