Tbh the is a point to what massivewaggers says, For best buck for power really 2.0 is the easiest way to go but if your not after massive power a 1.8 can be a much more rewarding engine.

Easiest power gain really is turbo as you can do them pretty cheap (a lot less than what it take n/a tuning to get to the same level of power) and thay can be quiet reliable But if you not after big power and just wanna abit more power and do things one stage at a time personally id stick with a 1.8 with looking at the engine rev better and faster, yes will have less power than 2.0 but will rev a dam sight quicker.

Either way if you want to stay n/a here's a list for you regardless of engine cc....
More aggressive camshafts
Head skimmed ported and 5 way valve angle.
Lightened flywheel
St170 spec exhaust system
ported and butterfly cut 2.0 t/b

That will see an easy 150bhp+ in both engines.