Depending on the Condition / mileage / MOT / History, the knocking is the big end shells ( On the crankshaft) depending on the strip down, crank condition, piston condition, valves / Camshaft condition, if the car has a good prior service history, myself would consider a rebuild, that would mean a recon engine in other words, the head would be skimmed, valve stems checked & new seal inserted, it's a lot of work, but as said, you can get a 2nd hand engine engine, the trouble with that Pink, it will take a day to take out & refit an engine, but you will not be guaranteed that's it's a good one, this can only be found when the engine is running, ask around your local area for prices, DO NOT go to the dealers, do you know the ownner of a small garage that would do the job for you? if so, expect the job to take about 2 weeks, the cost, in the region of £800 / £900. If you do decide to get it done, it would be cheaper to go that way than get another that you do not know anything about! History etc.