It is indeed a completely different car. The only thing they really share is a period in time when the LS2 was found in them both.
There are plenty of good specialists in the UK who can provide parts, and the boss of the Vauxhall VXR brand is a member on Pistonheads and very approachable for assistance.
The engine is the same as in a Camaro, Corvette, etc. So any american specialist can supply engine parts and expendables. Brakes are AP's, so bad and discs etc are easy enough to get hold of.