300HP is impossible for NA builds.
Like BoredNord said K24 bottom, bored cylinders, pistons, cams, and especially a tune are all necessary if you're aiming that high. These engines suffer with bolt-ons. It is more efficient if you go FI, if you're looking for the best $ per HP
http://www.8thcivic.com/forums/civic-si/300669-getting-300hp-n.html
Like BoredNord said K24 bottom, bored cylinders, pistons, cams, and especially a tune are all necessary if you're aiming that high. These engines suffer with bolt-ons. It is more efficient if you go FI, if you're looking for the best $ per HP
http://www.8thcivic.com/forums/civic-si/300669-getting-300hp-n.html