Then a Nissan V6 makes good sense!
Browsing Trademe.co.nz (a local ebay-type auction site) just now I came across a Mondial complete for NZ$5000 - which is quite astonishing. I believe Cascu on this forum uses these as a basis for 246GT replicas. The rebuild on that engine would cost more than your whole project probably!
Yes the Chapman car was built by Dave Brown at Classic Car Developments in Invercargill with bodywork by Barney Tansley of Queenstown. I visited them about 20 yrs ago and I'm sure they had more than one chassis in progress....
I'll try and attach a few photos. I wonder where the car is now?
http://tansleypanels.com/
best of luck with your project
cheers
Ross in Auckland, NZ