Zetec is the easy one, as so many people have done it, but still requires a bit of messing with mounts and things.

RS2000 is less common, but it's probably the one I'd go for! Bolts up to a CVH box fine, just needs a bit more thought with one of the mounts. Then just use the engine loom from a MK5 RS2000, spliced into the XR3 wiring.