head wise. normal stuff - cam, port, bigger valves. who does it is up to you
another option is to change the head, I think the starion boys do this a bit, certain model SOHC heads are fairly easy to fit and flow better.
have a read on austarion.
yet another option is to go twincam head..... but i wouldn't recommend it. May aswell go a whole VR4 motor if you ask me!
if you stick with the SOHC 4g63 cams wise, you have little choices:
1) Regrind. wade and camtech i think do the "biggest" regrind's. but you are limited to how far you can grind it,
and MUST be adjustable tappets. but cheap and easy to do (wade regrind cost me $60 or something)
2) weld lobes and grind them. Allows a better range of profiles, but last time I investigated this Wade cams
would not do it on a street car because the cam wears out much faster when done like this.
pretty sure this is also quite dear to do.
3) Swap your rocker gear and cam over from a sonata or galant (same model you get MPI from from memory).
these use the same port designs but use a hydraulic lifter, roller follower setup.
last time i checked you CAN get billet cams for these models, then get a suitable grind. Contact RPW for more info.
This is the track i went down when i had my cordia (I also tried a wade re-grind on adjustable tappets).
It will cost you a bit for the cam, but the rocker gear you can pick up from the wreckers for sweet FA, and its a straight
swap over.