D,
Just use a Fuego turbo motor and put the R18 pistons and sleeves in it. All the manifolds are made for you.
Personally, I have many renaults at home (several with the r16 motor) and I would not waste your time. Whilst they can make huge amounts of power with a light motor, there would be a huge hassle fitting a gearbox up to it, and they are rarer than you would think.
Just put in a Ca18 or SR20 - and save your money for the speeding tickets you will receive
