Automotive carpet is inexpensive and readily available online. Auto Custom Carpets CLICK HERE
is one place, but there are many others. A custom fit carpet for your Skylark sells for a little over $200.
The interior work is another story. It depends on what all you want done. I would say $3000 to $4000 (including the new carpet) will get you exactly what you want. That would be a headliner, kick panels, door panels, package shelf, and seats.