Vauxhall Corsa loss of power engine making loud noise on putting on power

ive just purchased a corsa n reg it had a high performence exhaust fitted and kn filter sounded great i drove it home but it kepted cutting out especially on low revs but a quick restart and it was ok the next day i filled up with fuel and topped up oil as i drove off the engine started making a loud clunking noise and i lost power i managed to get her home in second gear only on tick over it sounds like it was firing on 2 cylinders but ive replaced the spark plugs no difference but the noise isnt there unless i put in gear and put the power on