If it is deffo an S2 shell (not just a mk4) then it is still an RS Turbo...but would still be classed be a replica as he's made it into a S1 if you se what I mean. It *should* either be on the original S2 plate if he'd carried out part A below or on a Q-plate if he'd gone by part B as per DVLA registration rules....

In order to retain the original registration mark:

cars must use:
A: The original unmodified chassis or unaltered bodyshell or a new chassis or monocoque bodyshell of the same specification as the original supported by evidence from the dealer or manufacturer (e.g. receipt)
And two other major components from the original vehicle ; ie suspension (front & back); steering assembly; axles (both); transmission or engine.

B: If a second-hand chassis or monocoque bodyshell is used, the vehicle must pass a single vehicle approval (SVA) test after which a "Q" prefix registration number will be allocated

Part B says it all.... 2nd hand shell using original S1 cars parts: should be on a Q-plate.
If it is on the original S1 plate, then he's made it a ringer...

Deffo walk away mate imo.
If you want an honest S1 at the right price, I'd give the man with the NMS S1 signature a quick pm

As for the initial question: front & rear panels are indeed different between the S1 and S2 due to the differences between the bonnet/ headlights on the front, and rear lights on the backend.