Interchangeable rear diff?
 

I just recently got a1989 Mazda Rx-7 GXL. No engine, no transmisson, and when I got there I didnt have a rear end ao i assume the guy sold. I have a everything for it but the rear end and I'm running out of time fast. Does anyone have any ideas or suggestions of anything that can be interchangeable? I've looked into just getting another one for it but i can't find them anywhere I live and I'm not sure what to do. First time i have ever owned a Mazda so this is all new to me. Thank you for the help