114 knocking in head

[rhurbarb2003]

Just bought my girlfriend a 114 ,i was told by a friend it was a good car ,when i started it up to drive away it sounded like a diesel.The noise stopped and the engine sounded not so bad.So i thought id be clever and flush the engine,then a new filter and magnetec 15/40.too my horror the noise was just as bad if not worse then went after 30 secs ,i left it on tickover didnt rev the engine with the new oil first ,soaked the oil filter and done all the sensible things.im very angry that the guy told me the car was ok.i took the car knowing maybe because one of the oilways were blocked ..im not sure how to approach this guy who i bought it from..the car was so mint i didnt even drive the bloody thing.yes i am an idiot for doing so,but i went with what he said .so now i have a 114 engine 16v with a sticky follower or lifter not sure which one you call it.either i buy 16 new ones or can i change the one which is faulty?..where can i purchase these at a sensible price ...im a properitor of a car accessory shop but partco and the rest seem to be over the top....if anyone can help me il really appreciate it
with thanks rhurbarb