This is a fair bit of work, because the stock Android apps won't compile as SDK apps (i.e. a standard APK that you can upload to the market) because they contain lots of references to private data and APIs. Removing/fixing these references takes a while, and in one case was not possible.
But anyway, I have now finished (more or less) three of these apps!
Calendar: Market, APK
Launcher: Market, APK
Messaging: Market, APK
All tested on Galaxy S2 and seem to work.
NB: As in the Apex launcher, the widget-adding experience is not totally correct. This is due to a stupid permissions thing. More details in the market links.
If you like these, consider donating to me! I may do more if anyone actually does... but I remain dubious!