The one I saw on TV is using the 6 litre Jaguar V12 (from the XJS-R and about 360bhp).
The kevlar-composite body versions are available with the 4.2 litre Jaguar V8 (I know that standard road versions of this are 300bhp or 400bhp supercharged).
They are currently building two for customers using a Ferrari V12 motor.
The web-site is definately worth a look.
Vanwall Cars is trying to find a US distributor.
So if you've got $90,000 dollars spare...