That's quite an open question, and it very much depends on how much power you want, and how much you want to spend.

The easiest conversion is to fit the 2.0 Zetec from a Mondeo, which will give you around 130bhp. You can keep the original gearbox and wiring loom etc with this one, so it's the simplest and most cost effective way to get a decent increase.

You could fit the engine from the RS2000, but that really requires the gearbox, subframe, wiring loom, hubs etc, so is much more involved.

Plenty of other options that would require a little more work. The ST170 engine is basically just a Zetec, so that would fit. The ultimate is probably the MK1 Focus RS engine, which is also a modified Zetec, so physically fits quite easily, but would require an aftermarket ECU, but would give loads of tuning potential.

You could fit an RS Turbo engine too, but that's perhaps a bit of a backwards step these days.