a '91 Orion could be a mk2 or mk3 so will really depend on which one!

If its a mk3 its a pretty straightforward job as its straight out the Escort and into your Orion, taking over full loom, injection etc.

if its a mk2 its harder cos the mount on the inner wing is totally different. I did one of these a few years ago into a mk4 XR3i and biggest bother was the mount. A metal fabricator I know ended up making a mount based on a mk4 CVH one and that got the engine in (bolts straight up to gearbox in your car with Zetec flywheel) but we had to abandon the project due to lack of premises. Think we would have also had problems with alternator set up as the PAS pump was removed.

Having driven a couple of RS2000's and having a few Zetecs now (I run a 2.0 in my mk4 Escort daily) I would opt for the Zetec everytime as I think it revs better.

In saying that, the I4 from an RS2k makes an awesome noise