I have a Frigidaire in the wall that never did cool the room. Fan motor not up to
Speed. Because it has a slide out chassis. I'v been told I could slide in another AC into
The cabinet, without tearing it out of the wall. As long as they were the same size

Do that sound correct

Although a "slide out chassis" may under certain conditions cause some losses (if not properly installed and isolated), it should for sure cool the room - if it's BTU capacity equals the room's requirements.

But a system that "never did cool the room" is something entirely different!
You better have an expert checking your system soon, and sorts the problem out!

"Do that sound correct" : no it does not , and in more than one way !

:D