An Eagle SX4 has the exact same seat as a Spirit. Exact same.
A Spirit will have a different inboard seat slide rail than an SX4, but it takes about 20 seconds to swap them.
Four little 1/2" drive nuts hold the slide rails to the bottom of the seat. An SX4 uses the exact same slide rails as a station wagon.
A non-power Station Wagon wide body seat will bolt directly unto the slide rails of an SX4. They are totally interchangeable. The power seat frames are different and they have a different set of holes in the floor, but those can be easily installed as well.
A 1981 SX4 and a 1981 Spirit will have a seat that does not recline. Those frames are total junk. They are extremely weak and develop cracks in several places. A 1979 Spirit has the same frame, but different welting work in the fabric.
A 1982 or 1983 Spirit or SX4 got reclining seats that are considerably stronger than the earlier version. The handle and reclining mechanism is exactly the same as a Station Wagon seat, but it doesn't have as many teeth and doesn't recline as far back. It is also quite prone to cracks. They do not crack as severely as the early non-reclining seats, but nearly every drivers side frame in existance is cracked already.
If you see your drivers side sitting a few inches further back then the passenger side, the crack is already developed to the point of needing immediate attention and repair. Every SX4 I own has this problem in various stages of development. Its not a question of if your frame will break, but when. The reclining seats are certainly worth repairing and the repairs are relatively simple. I won't waste my time repairing the early non-reclining seats.