Since releasing the X11/TCL/Tk version for OLPC, we've stripped out the user interface, cleaned up and refactored and doxygenated the city simulator engine code into C++, and wrapped it with SWIG so it plugs into Python and other scripting languages. It's not as "authentic", but it has more comments and is easier to read than the old C code:
https://github.com/SimHacker/micropolis/tree/master/Micropol...
Here's a talk about it:
HAR 2009 Lightning Talk Transcript: Constructionist Educational Open Source SimCity, by Don Hopkins
http://micropolisonline.com/static/documentation/HAR2009Tran...
I got hooked on computer games by playing Zork at MIT-DM, and it really blew my mind to finally be able to read the original Zork source code in MDL. Reading the source code takes you backstage behind the scenes of a world you visited years ago, like the Disney's Keys to the Kingdom Tour!