I'd suggest getting on the Facebook scion exchange group to look for quince and medlar scion.
But if you have plenty of hawthorn, I'd keep grafting pears on them and I'd wait to worry about delayed graft failure until you see it happen. If you get any pears off a tree before the graft fails, I'd call that a success that just cost you a little of your time. Keep grafting a variety of pears and see what happens. I'd do bark grafting on 3-6" hawthorns if you have them. That way you have a few more years before the top growth of the pear would exceed the rootstock under it and maybe cause of the top of the tree to break off.