I'm no seat cover expert but I have done a few. Did the seats in my el Camino and my Corvette and they turned out nice. As far as replacing the foam, I think it depends on how sacked they are. I think you get a better result if you replace them but it's not always necessary. I personally found that it is a lot more difficult to fit the cover with new foams as they are fuller (is that a word?) and less forgiving. And yes, you can repair the foam if you use some with similar density. Cut to fit and glue with either foam glue or mild spray adhesive.
If anyone on here is an upholsterer, they might have better advice.
