MiKTeX source code is available via anonymous (pserver) CVS.

To get a copy with a command-line CVS client type:

  cvs -d:pserver:anonymous@cvs.miktex.sf.net:/cvsroot/miktex login
  cvs -z3 -d:pserver:anonymous@cvs.miktex.sf.net:/cvsroot/miktex co miktex

When prompted for a password for anonymous, simply press the Enter
key.

To browse the CVS tree, direct your HTML browser to
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/miktex/miktex/

If you're not familiar with CVS, or the above instructions aren't
clear, the following link may be helpful: http://www.cvshome.org.