Thanks for the comments.
I've been in touch today with Andy from Saturn. He told me the DOHC fits under the new bonnet, (enough clearance for the camshaft cover) but you still have to modify the inlet manifold. Andy directed me to the building diary of Enoch who fitted the DOHC under his Saturn bonnet :
(
http://www.deepfolly.co.uk/8.html).
This modif is a hell of a job... so I still don't know what to do.