I can’t answer your question directly (hopefully someone else can) but the regular S-Type 4.2 and 4.2 supercharged mufflers are different part numbers.
Whether they fit interchangeably, flow differently, or sound differently, again I can’t say. But they will look different. The 4.2 normally aspirated muffler has a slant-cut tip and the 4.2 R mufller a larger stainless steel tip with no angled cut. So if you did fit a 4.2 muffler, it would make your S-type R look a bit like a hacked junkyard special with mismatched tips.
The tips are welded on, so to get a matching appearance (if they did fit) you’d have to cut them off and re-weld them. Unless you can do this yourself, you might be better off just buying a new one (which are still available) though they aren’t cheap.
If you don’t mind the cutting and rigging different parts a good muffler/exhaust shop might be able to fabricate something at a decent price, hopefully transferring the old S-type R exhaust tip to the new aftermarket muffler to maintain the original appearance. (or go all out and replace the entire post-catalytic exhaust system with a stainless steel aftermarket one!)