Intake, exhaust, cams and lighten the crap out of it.
To be honest, don't expect it to be a rocket. Too many young new guys (like myself at one stage) came on here and thought they were gonna turn the car into a beast by doing a few things and it simply doesn't work like that, you will see a gain, but it will make you bored soon after.
Make the car into a handling thing, bit hard with the u12 as no-one does much, but after seeing some stuff recently, the use of branded parts in racing apart from motors isn't there as much in chassis design.