2bars-
Do I understand correctly that you want to replace your original SPF rear rotor with a different one that has a 2 3/4 inch center hole (so it'll fit over the new hub)? That seems like a needle in a haystack situation to me. Gotta match rotor OD, thickness, and stud hole spacing to the original. Have you considered either enlarging the center hole on the original rotor or machining down the flange on the new hub? Please understand that I'm not recommending either of these because I don't know what the effect would be on mechanical integrity. But I'd at least look into it. Other options: Olthoff Racing has some beefed up rotors and hubs that might be worth looking at (
Parts and Accessories), and Hillbank may have a replacement SPF hub in stock by now (they didn't have any 3 months ago).
Good luck and please let us know how you resolve this!