Zetec engine identification

[Shorty]

Last week I picked up a zetec engine from a 2001 focus (supposedly 2.0)
I didn't think of looking on the side of the block to confirm as the car that its from comes up as a 2.0 when searching for parts etc with the reg.

I have just looked at the side of the block for the engine number which I can't seem to find and the large number on the side is 18 so i'm guessing it must be a 1.8.

I can't quite understand this, I don't mind if it is a 1.8 but why would this happen???
Mabey the owner trashed the 2.0 engine and as 1.8's were cheaper they swapped for 1.8???
I have the ecu and most of the loom. Would it have been a straight swap to change the engine and ecu?

Any ideas?