Lol @ Chip (think isee what Leigh is saying though)

I think a Unichip would give you your best chance of mapping your 1.8 ecu for boost,

the frs pistons will not fit into a 1.8 block this is due to bore size and i dont beleive it is possible to over bore the 1.8 to suit,

the heads are specific to each engine, the 1.8,2.0 and RS engine all have diferent heads, this shouldnt be too much of a problem, but to be honest the main point you should look at is the gearbox in the 1.8 focus its only the IB5 which is not the strongest by a good way, the 2.0 came with the stronger MTX75 box, and the RS has un uprated version of the MTX75

if i was you id go for a full FRS conversion, so many little things you need to do to get boost ona 1.8 the time and money it will take you are best just going for a full conversion i think,