Hi and welcome to TorqueCars, nice to have you along. I thought the ZR was on a similar chassis to the 25, just with better quality components bolted on. This is something Stamford will know.
How about an engine swap to the 2.0 Turbo from the older 220 GTI? The chassis and engine bay you have is almost identical to the older 220, the shell was redesigned and bolted on to the old chassis - typical Rover.
A lot depends on your plans for the car and how much you want to spend.