ok mabee i got out of hand.

if u want a slight gain without breaking the bank and staying original u might try buying the 4g63 head it will bold onto the 4g15 block without any problembs, my only consern would be timing so u may want to get the crank pully off the 2.0 aswell just in case. adding an exhaust prolly wouldent be worth the money in this application unless you wer spraying or boosting the car.
put an intake on it, stock throttle body is accualy fairly large 2mm larger then a stock 4g63t motor so it should be fine.
stock 1.8l makes 87hp i belive and the dohc 1.8l that comes stock in some hyundias made aprox 114hp thats a 27hp gane from a head,that shouldent cost u more then 200 bucks even after a timing belt the gear and some parts cleaner. this will prolly make a considerable amount more then a stock n/a 2.0 because the 1.8l uses the same block with a slightly smaller bore and a lower dech hight witch will give it higher compression.
from here if u want more power witch u prolly will u should try to find a 2.0l turbo block.