Crin is a filling material that is often used in classic furniture. It is an indispensable material if you want to reupholster classic furniture in its original condition. The official name is crin vegetal. It comes from the leaf of a dwarf palm from North Africa. The leaf is cut, and later curled.
If you're going to the upholsterer anyway to check out the fabric, why not get them to rebuild the seat? You already have plenty of projects and they have the skill sets to both make it look new and more importantly perform better than new. They could use foam (memory or regular) or older materials. I made the same choice on my motorcycle seat years back and it made an enormous difference. Don't YOU deserve a good comfortable seat? It's where you spend most of your time using the vehicle.
Just read in the paper today that my state has started denying registrations of all vintage micro cars and recent Japanese Kei cars and trucks. Think they said emissions and safety were the primary reasons.