Best bet is to get on the Skyline owners forums most are free and have plenty of tuning sections and owners whom have modded up skylines including non turbo versions.
some mods would be an panel air filter with a 3" pipe from the front bumper or a bonnet scoop which feeds more air into the air filter box many use apexi air filters,a decat pipe fitted,iridium plugs,decent oil change with expensive oil.
Best bet is get on the forums for the exact parts you will need and the best places to get them from.