i' ve read somewhere that the oil breather function is to release pressure from the engine.

what are the pros and cons if I install an oil breather filter to replace the pipe?

and what type is that filter because I can't find one designed for that (in my country) so maybe I can improvise with something else.

nothing will happen as that is what an oil breather filter is designed for and thats how you fit an induction kit.

heres the one i got it fit my 1.2 16v engine. but just measure the diameter of the pipe and get one to fit.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=120342617279

Venting to atmosphere is illegal.

The gasket leak isn't caused by the breather at all.

Yep no back pressure provided by breather system so wouldnt affect gasket. Wouldn't really affect engine at all if there were no pipes, they are just routing the crankcase gases to the filter, except that its an M.O.T fail to vent to atmosphere (and you would smell it through your blower.)