See if you can get new foam first, since the old stuff has degraded it will continue to fall apart.
You will want something like the 3M 77 Spray Adhesive and spray both sides let it tack then attach together.
Foam is actually in good shape, I think the boot was made in1995 has a guy dated it and signed his name. Just a little tear. Its a little oversized; but I will have to see h ow it looks before cutting any out.