Putting the engine in is the same for any rwd zetec, fit a sump, water rail etc lots are available and use cvh engine mounts and make up an exhaust, you also need to consider the power steering or lack of and the alternator.

You need to decide on the gearbox, t9 and mt75 will only except a 220mm clutch a t5 or hybrid mt75 will allow a 240mm one to be used then get a flywheel to suit, could also go Mazda probably.

The inlet manifold will not fit so a change to a plenum, throttle body's or carbs will be required, depending on which water rail is used you may need to gain some clearance at the rear of the engine, and fit a cosworth radiator. Only difference with the st170 over a standard zetec is if you wish to retain the vvt then the engine management has to be able to control it. If the car came with a carb fed engine and you go to fuel injection then the fuel tank will need replacing.