If worse comes to worse & you are not able to un-stick your original rear brake caliper. I think I have one laying around here. It's not perfect from what I recall. It was on a bike that was dumped on its right side & the brake caliper has some asphalt grind on it. If you are interested, I'd let it go cheap.
Good luck
J.
PS. I've heard that there are other calipers that interchange. Unfortunately, I don't know what they are.