Can you clean the trash out of it and make up a plate from some scrap steel? Anything to keep rubbish from getting in there again making it bind up.
Probably the best way to tackle this would be to remove the whole seat / slide assembly from the cab and swamp out the area. Then you can separate the seat from the rails and clean / lube those up so they work freely.
As for the cover, that might be doable if the base material is still good, but would probably cost more than a replacement seat from
Surplus Center. When you get the seat off the rails you can determine what bolt mounting pattern you have and match it, as SC is quite good at providing mounting details. Then when it's all back in, get some flashing from Home Depot or equiv. and make some shielding to keep debris out of the track.
Another option that might be less expensive is to get a whole bucket seat assembly from a wrecking yard and drill the cab to accept it. Most only have four mounting points, and you can grab a passenger side that will usually be in better shape. The drawback is that they are not designed to be in exposed locations so in an open cab it will weather and act like a big sponge, which is basically what it is until covered in vinyl.
