So i have a fairly big crack on my exhaust header, i was wondering what are the effects of this? Performance wise does it affect it or long term effects etc. is it worth buying another one at a junk yard?

It effects performance and fuel mileage. Yeah, fix it definitely.

and some say co poisoning, if it leaks too much

It will mess up your fuel trims and you will start to see a downhill slide in fuel economy. O2 sensor will keep adding fuel thinking that the air coming from the crack, is coming from the engine instead of outside. Get her fixed. Maybe time to add a good performance header?

If you got an ST. I have the stock upper exhaust manifold. Also got a useless 4-2 exhaust for the st. Its missing the 2-1 piping.