The 1.6 Focus engine is completely different to the RS so you can't use the manifold and it's not a strong engine so you're very limited to how much boost/power you can run. People have done it in the past (think van aaken) but they almost all went bang pretty quickly.

There is one company doing it that seems to be having some success if want no more than 200bhp http://www.oc-motorsport.co.uk/

It's technically possible but it's a tricky engine to tune, especially as if you go mental tuning it going the N/A route it will never have enough torque to haul around the relatively heavy Focus. Then there's the gearbox which is as weak as a kitten with the flu.