Easy thing to do is use ST170 sensors as they will be correct distance away from the ring to give correct pulse.
I am not familiar with the focus or St in this way, but it would be easy to see where the ring is for the abs just take out the sensor, I would hazard a GUESS it will be on the driveshaft, abs sensor values will be the same on normal and ST so just new sensors will correct it.
You will know the sensors trigger ring/wheel by a series of uniform serations ie bits missing, lots of flat teeth with small gaps in between.
tabetha