Built head on a stock bottom end

Hello everyone im looking for some pointers on whether or not to build just my head on a 09 civic si and keep the bottom end stock. Or do i just rebuild the hold engine top to bottom. The car has 140,000 miles and im looking to make decent power. Right now it has intake, full exhaust, 72mm throttle body, and hondata. Im still new in the honda game and i just want some pointers to help me figure out to what i can do to gain some power.