The 2.0l 8v is not really worth spending money on. You will put probably £1000 into it before it's even got half a chance of beating a 16v.
But if you insist, a high lift cam, a modified airbox with a panel filter and cold air intake, exhaust and a good service will get you 135ish hp. Mine had got 138hp with the above, but it had nothing on a 16v.