i had my new exhaust system fitted today by Tony Franks. The system was made by Janspeed as a replica of the one on Tanya's Tommy. I asked Tony about the Grand Prix exhaust back box as it looks similar to the original S pack. He said it is similar but not identical as the injection boxes were a bit flatter.
I do not know if you have seen the Cooper brochure from around 1995. One of the conversion kits advertised was the Super Kit, MCC1275SS. This was the same as installed in the Grand Prixs. There is a good illustration of the kit and I think the back box was the same for all the SPi S kits. The funny thing is that either side of the illustration is one of two halves of a photo of Mike and John Cooper and Tony Franks having a conversion around a Mini engine bay.
I know from the service record of my car the original S pack back box only lasted 5 years and 10k miles so I am afraid it is unlikely that you will find any originals around now. But there are enough owners with SPi S conversions around to have them remade providing you have a minimum order of ten. RSP owners seem to be more of a community particularly with Tanya's energy and enthusiasm. The boxes were originally mild steel but we opted for stainless steel in the remakes for a longer life.