This is a big-ish job depending on how far you want to go.
The obvious things are engine mounts, exhaust,cooling system & engine management system.
The Zetec will bolt to your existing G/box & propshaft.
The Zetec intake system will not fit.
You will need an intake manifold that will permit the use of either 2 twin choke injector bodies or 2 Weber carbs (which keeps the fuel system simpler with the addition of an electric fuel pump).
Look at all of the previous threads on this subject, there is, as people have said, a lot of information available on this Forum.
I've done the job in an Elan & enjoyed the challenge, which it was at the time, due to the lack of information.
The concept of turning an Elan into a modern car purely by the addition of a relatively modern engine is totally misconcieved in my oppinion.
Spyder replace alot of other parts as I did, which does a lot to get the major mechanical components up to a modern standard.
However do any of these parts cause day to day problems in an Elan?
I think not, apart from the odd oil leak.
Look at the Forum questions on electrics, windscreen wipers, window motors, switches, headlight vacuum systems etc.
These are the bits that hold our Elans back in the 60's-70's era.
Sorry guys, I always have to add my three penneth when the Zetec thingy crops up
PS I do love my car the way it is. Pure adrenelin
John