The later RS2000s have PATS -Passive Anti Theft System, which is a bit more difficult to get around, so it's just easier to use the engine loom and ECU from a 1991/2 RS2000 which doesn't have it.

I think it's a good conversion for the MK3/4 Escort. Engines are cheap enough, 150bhp out of the box, and plenty of tuning potential, despite what people say. A couple of slight tweaks can get you a useable increase.

As far as I know (and this isn't gospel by any means), you have to modify the offside engine mount, and you can then use a normal MK4 gearbox and flywheel set up quite easily. Fit the RS Turbo box to get a limited slip diff. Use the MK5 loom and ECU, injection tank and lines. You'll need to sort alternator, but if you're converting to PAS, you may be able to use all the RS2000 stuff in standard layout.

There's more knowledge around about the Zetec conversion, but the RS2000 lump is very underrated, and 170bhp isn't particularly difficult to achieve. Even the standard engine should make a lightweight MK3/4 Escort move pretty well I should think!