st170 bottom end means it's the later zetec E engine. They have steel rods in them, not forged but still strong. I don't know a lot about the zetec conversions, but it's certianly possible. If it runs ok, then i wouldnt be worried about it. Be more concerned about the rest of it. THe bodywork, the sills, the arches, floor, the arches. U can get good CVH engines for 200 quid which will outlast the life of the car. Ie, if the shell is solid, then dont worry too much about the engine.

Rick.