You would have to go to the "const g_mItems". There, you would modify the options that will appear in the menu and their price. Then, you would go to the menu handler, which is here "ShopMenuHandler". The only part that you need to modify is the "switch" function. It is zero-based, so the first option in the menu would be "case 0:", the second option would be "case 1:" and so on.
I hope I helped.
EDIT: If you are going to remove the noclip, invis or godmode, also remove their "remove_task"s from the "Event_HLTV_New_Round".