Careful with the stickers, as some of their adhesive can damage the helmet and reduce its effectiveness. They are fine on the peak but not on the main shell
Get a professional painter with the correct airbrush kit. More expensive than stickers but how valuable is your head? (some more valuable than others !)
I tested a helmet against a concrete pylon at over 115mph (the door post did not absorb much energy)…and am glad it was effective. Would hate to think what the consequences would be with a bad / cracked / brittle shelled helmet.
Spend the money and keep safe