Seat Foam

You wouldn't necessarily have to go out and buy the
burlap unless you wish the bottom of your seats to look original.
The burlap's role in seat construction is a method to
keep the foam from pushing thru the springs while being compressed.
You may have something suitable even better than
burlap laying around. Most any non stretch fabric made is a better choice since it is much tighter woven. An old bed sheet or tarp for example could be used.
The trick is wrap and hogring the perimeter wire frame stretching fabric tight.
Don't forget to ring every spring to the fabric to keep them from spreading apart from each other. When the springs spread the padding is easily forced thru
giving no support.
As for the foam use some high density for the seat
cushion replacements. Softer foam is better suited for the backrest. Carve foam into shape with a loose hacksaw blade or bread knife.