Yeah, fords not like Honda who makes every block the same so tha any head can go on it. My brother for example is thinking of putting a 1.8l head on his 1.5l block.. will it work you ask.. you freakin bet.. cause they're all the same.... any way.. Nope dont think so
Yeah its common to put a B18 head on a B16, or vice versa. They both have about the same flow characteristics, but the B18 has a naturally more aggressive VTEC engagement.
Putting a B18 head on a the old 1.5L (D15..I think) I'm pretty sure wont work. I know for a fact the D series blocks and B series are different. I know that in some cases you can get crazy with frankensteining...but not too crazy.
Like-wise, all the old F20 and H22 and F23 are all completely different and aren't interchangable.
As far as the Escorts here's the most complete answer...
The GT 1.8L is a Mazda BPD8 motor. Designed by Mazda.
The 1.9L SPI is a Ford designed motor...hence why its radically different. However the Zetec series motors are a Mazda engineered motor, but actually built by Ford...so there's more in common with the 1.9L and the 2.0L
second note, if you actually have an old 1.6L 323GT or Miata 1.6L you can raid an EGT for parts to upgrade your 1.6L. The BP6 can use the 1.8L fuel injectors as a good upgrade.