Son, I see your post is from a few months ago and I am hoping you already have a solution for your planter. Did you manage to find the parts you needed for your food plots?
To answer your original question, a John Deere hopper will not bolt up directly to your Case 900 row unit without some heavy modification. I remember the mounting systems were quite different between the old company machines. Your best chance for original fiberglass hoppers is to get in touch with a reputable tractor salvage yard, as many of them keep parts for planters of that vintage on hand.