if you want to keep it a 1.8 then its going to have to be custom pistons, standard off the shelf pistons for a 2.0 will take a 1.8 to a yep, you guessed it. 2.0. Also, not wanting to tread on your feet paul, but a set of eagle rods imported from the states are a lot cheaper than £700, also AFAIK the 1.8 and 2.0 head have exactly the same castings and it would be much better to weld a zetec flange on to a ERST manifold. Good luck with this mate, just accept its not going to be a cheap and quick build