My Ford Puma 1.7 1999 V reg has been misfiring lately. Over the last few days it cuts out when I brake and misfires at low revs. When idling I have to keep revving to stop it cutting out. The car seems noisier and bit more 'shaky' than usual. A garage seems to think that one of the cylinders in the engine needs replacing. They quoted £800. I can pick up a secondhand engine for £250 but the garage says I need to get one from Ford Direct, as a secondhand one would not work in my car. Also, under the bonnet I see that the person I recently bought the car from has fitted his own performance air filter which is covered in oil, and this is leaking under the passenger side at the front. Could this have anything to do with the misfiring?