I'm not sure what an SVT Zetec is. I notice there are Focus ST 170 complete cars available round the £500 mark in the UK. Early noughties vintage
Buying one and breaking it might be a cheaper route to 170+ BHP. It says Duratec on the cam cover though. I'm guessing that the level of difficulty would be similar to fitting a Zetec from the same era of car.
Not everyone has space to break a car, but it's a route to a very cheap engine once you've sold the spare bits - seats, alloys, brakes, suspension, etc.