Yes the ST170 head will bolt onto a normal 2.0 block in a Focus if you change the middle cambelt cover/engine mount and alternator bracket. Then how are you going to run it? The engine management has to control the VVT, DSI, system and cope with the completely different injection requirements, fuel pump control, and sensors.

Many people have used a ST170 head as part of an engine build, but that usually with custom manifolds and stand alone management. Otherwise a complete engine/loom/PCM swap, though its not really economic.