You can reach 300hp n/a, but its gonna be a lot of work. Never impossible though, seen a n/a civic crx run 11's all day and all motor off a 1.8L and no issues.
So, it is possible, just a lot of building and parts, but boost is a cheaper route. I would rebuild the bottom end first for n/a, then work off valvetrain, without your external engine mods (intake, header, exhaust) you can achieve 300hp though.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille