A discovery rad would fit in there easy. They come as a four core standard with drop in pegs for ease of fitment and measure <just pulled one out of the shed so this is right> 540mm wide x 450mm high x 75mm thick. That can cool a 2.5 td pulling 2 ton of 4x4 so it would cope with just about anything in a roadster. it also has the oil cooler in the tanks so thats another thing out of the way. A decent second hand unit would only set you back £30. They also have an intercooler that might be usable .
https://www.firstfour.co.uk/item/217...bly-for-200tdi
http://www.google.co.uk/imgres?q=dis...:429,r:1,s: 0
Bob