Will the stock seats out of my 1990 Honda Accord work (Fit) in my 1987 Honda civic?
Not Likely. You can get a tape measure and see if any of the measurements are the same.
Try it. Worst case, you’ll have to put them back in their original places and you’ll be out nothing.
One idea, visit a used auto part recycler. Most of them have a database for part interchangeability. When I visit my local pick-and-pull for example I tell them what part I’m looking for a particular car, and they give me a print out of all the makes/models/years where that part is compatible.
Take a tape measure, eyeball the seat tracks, and do some measuring between mounting bolts for comparison purposes. Even if not a direct swap odds are a little improv will do the trick.
Many years ago I had an old Subaru whose seats were killing me. I happened to run across a guy who had an old junked out Chevy Citation sitting in the shade rusting away. He said I could have the bucket seats free of charge if I could use them.
I bolted the Citation seats onto the Subaru tracks and voila. My spine never quit thanking me. If a Chevy will fit Subaru then surely a Honda will go into a Honda.