Larger displacement of the same engine shoulden't be to hard, i.e. the 1.4 or the 1.6, I would personally go for the latter, as your current engine and the 1.4 do in fact have the same power output, although the 1.4 does have more potential in it.

The 1.6 is the engine which is fitted to the "Zetec-S" sporty Fiesta offering, and is also in the Focus, Cmax etc.

You probably need to change the ECU though, as the mapping will be different.

Martin