Is it permissible for the shroud to be sewn, or must it be unsewn like the clothes of Ihram?

There is no objection to the shroud being sewn, unless the deceased is a man in a state of ihram for Hajj or 'Umrah. In that case, he should be shrouded in his ihram garments, based on the Prophet, peace and blessings be upon him, who said concerning the muhrim (person in ihram) whose camel broke his neck and he died: "Wash him with water and lotus leaves, and shroud him in his two garments, and do not cover his head, nor cover his face, nor perfume him; for he will be resurrected on the Day of Resurrection, reciting the talbiyah."

The sewn garment that is prohibited is that which is tailored to fit the body (like a shirt or trousers), not merely something that has stitching.