How can install stock Android 4.0 on my rooted Galaxy S2?
There is no official AOSP* rom for Galaxy S II, but there are many custom roms of ICS available. I'd suggest trying Cyanogenmod 9 nightlies, which are not stable yet, but usually work pretty well. You can find more information from the Cyanogenmod Forums, specifically here.

Note that the nightlies come with no guarantees, use at your own risk.

*AOSP: Android Open Source Project, also known as stock or vanilla.