Depends if your looking to make small changes or big changes, You could add and air filter and a exhaust but its not gonna make much difference or,,,,,,,,,, u could go the full whack and start an engine conversion. Your choice!!!!
I put a panel filter and drilled the air box on a 1.6 16v Ecotec and it gained 9bhp. I know this isn't realistic, but the book figure is 106bhp on the std 1.6 and it rolling roaded at 115bhp.
Ours was low mileage, only 51k, but i converted it to a 2.0 16v (c20xe) with 92k on the clock. Is much better now apart from a few niggly problems with PAS and Aircon pumps not fitting. Also needs the cat removing to help with breathing.
The main problem with the 1.4 and 1.6 engines is the amount of bends in the exhaust run. A nice 4-1 manifold and a Mantzel Power box could make the 1.6 or 1.4 livelier.