Pretty stupid, you will probably now face frozen carb, lowered MPG, some running problems and lack of power at times. When it is winter carb will more likely freeze and problems will be more pronounced than at summer. Sorry
That is why there is intake air preheating in carburettor engines, even it can be rather restrictive it is better than tradeoff with better flowing airbox.
Even any theoretical performance benefit from such modification or K&N filter, is really minimal, sure one can maybe get such increase that can be perhaps felt, maybe even measured with clock if one tries enough hard, but there is no real benefit, even less now with those problems waiting.
Manifold matching, headwork, cam and rejetting carb would be methods to get some real benefits, also exhaust manifold and exhaust can make difference, loud exhaust does not equal more hp, however.
Sorry, you did ask what we think
It is really bit of pain to get more go for 1.4, one is to have standard not customized motor to run as it was meant to from factory, there can be 5 to 10 hp missing just simply because of car needing good tune up, word tuning used to mean making car run correctly by setting ignition, fueling etc. correctly, but it is different today I guess.
Now probably we all have done things at beginning which have been more work than gain, also there is saying that if your tuning does not loose more than 20hp, then it is success
Oh one more thing, often increased sound gives false impression of improved performance, only clock will tell how it really is.
One of not so difficult and that has some real benefit is matching manifold to head, this is done so that intake port (hole in cylinder head where to air/fuel mixture from intake manifold enters to cylinder head and where from it enters to valve and there to cylinder when valve is opened) and hole in intake manifold are made to be exactly same size and at same location, so that there are no any 'step', I have heard this giving quite lot of improvement on some motors. Same is of course needed for exhaust manifold.
With carb cars there is then added challenge, when you make any change that affects airflow, let it be even cutting holes to airbox, K&N filter or matching manifolds, there is then need to do jetting for carb, so one can get maximum power out from changes, so all changes you make are just opening possibilities, but to use these possibilities fueling is needed to be adjusted by jetting the carb.
So when you open air filter box you increase air flow, but not the fueling, this means that you lean out air/fuel mixture which will decrease the power. Remember that 14.7:1 is optimal for fuel usage and power, also catalytic converters work best with this, 14.7 parts of air and 1 part of fuel, but if you would like to get more power, you should not lean out, but enrichen the mixture, in turbo applications I have heard that even 8:1 would be usable and would give best power, so you would set carb so that it is bit richer at full throttle than it is standard to get bit more power (again tuning, jetting carb), not leaner which opening air filter box has now done.
There is of course some shortcuts and cheap tricks, but doing those one will often end up motor that works well in one condition but will works much worse in every other condition, so it is not very good choice.
With fuel injection things are lot more pleasant as those are able to adapt and one can easily convert fuel injection to programmable, so that it will adapt to changes with few clicks of mouse, a lot easier than working with carbs where to make change to fueling you need to change parts, but do carb setup right and it is really nice to drive. Also with fuel injection you will not get that carb icing as there is no carb to freeze, so there more cool air will mean more power as most fuel injection systems compensate for more airflow pretty well. I had fuel injection and made kind of 'ram air' and I could see from laptop screen that it did increased pressure at intake manifold which means more air entered to engine as speed did increase, very small effect really.
I don't wish to turn you down, just to introduce some basic aspects that are needed to examine and learn to get some real benefit, it is true that there will be bit of work and some things are quite expensive too, however end result is really nice. Standard 1.4 had something like 70hp, I would think that 80hp is not going to need really a lot of work, maybe even matching manifolds and jetting carb would be enough, freeflowing exhaust and maybe tubular exhaust manifold (is there such for 1.4?) might help to get even few hp more, maybe even near to 90, with cam 100ish? Headwork and valves might be quite high, perhaps even too much for standard carb, there is only so much air that can go trough those barrels.
Just some thoughts there, but doing the hard work is going to be really worth it, at some point you can really feel engine to wake up and star to live, but it needs that long walk to make it really happen, that is at least what I have found out.
will all need better examination. which brings me to the next point - thanks again for the info jtbo, really helpful and points alot of things i didnt give much thought. i will do the checks you suggested to see what will be "doable" will probably get it on stands tomorrow after my driving lesson.