Which 1.8? The 99-2000 ES's had the FS version of the 1.8 liter is called the "FP-DE". It is basically a destroked version of the 2.0 found in proteges today. 85mm vs 92 in 01-present cars.
Mazda 1.8 DOHC
125HP
108ft/lb torque (i think the intake tube and box are slightly different from the mazda engine. There might also be some minor differences in the ECU)
Everything is the basically the same except for the valve cover which would say Mazda instead of Kia.
From the motor i have seen in person the 2.0 valve cover looks alot wider than the 1.8, i am very sceptical that it would fit. Plus the fact that the 2.0 now has direct port ignition just makes the likelyhood that much less.
Anyway i have abandoned this project, i have bought a motor for my Probe I a going to build up instead.