The RS1800 already has the bigger MAF etc. To get to the 130PS from the 105PS, they changed the MAF, cams, throttle housing, inlet manifold and ECU, plus fitted oil spray jets under the pistons.

There probably aren't many easy bolt on mods really. You could fit a different exhaust system, a decat if you can get away with it, then look at different exhaust manifold options. Ashley do one I think, you could adapt an ST170 one, or there were a couple of other options around at one stage, been ages since I've played with a Zetec lump. This MIGHT gain you 1/2bhp, but that might be at the sacrifice of torque.

Wilder cams, a mild head skim to raise the compression, plus a proper, mapped ECU might gain you a little bit. Add throttle bodies and you might gain some more. Something like that may be about 150bhp, but that's a guess, as I don't have much experience of 1.8 tuning.

The most cost effective way of increasing performance is probably to find the 2.0 engine from a MK1/early MK2 Mondeo and fit that. You can use 1.8 parts like the sump and oil pick up to convert it so it fits, along with the Fiesta engine mounts and manifolds. The basic engine architecture is the same, so it's just a case of mix-and-matching parts to make it work.

I've had loads of Escorts over the years, including a couple of MK6s with 2.0 Mondeo engines, and it makes a big difference over the 1.8. It feels a fair bit punchier thanks to the torque increase. And if you apply the modifications mentioned earlier, you'll get more power out too.