Depending on which Fiesta depends on how much work is involved in fitting it.

A base model 1.1 with a xflow in it will need more work than one with a CVH, and then the 1.8 XR2i's came with Zetec's to begin with.

I take it you mean the RS2000 I4? Yeah it's a good engine, but limited tuning capability, so better going for the zetec and the zetec is alot more common as well