Get opds.py from the source code. Change it as needed, then put it into the right place in calibre's installation directory. If you have opds.pyc there, you can leave it. If you are overwriting opds.py, save a copy first for safety.
What problems are you having with running from source? You shouldn't need to compile anything. Download (easiest is with bzr) and put the source tree somewhere, set the environment variable CALIBRE_DEVELOP_FROM to point at somewhere/src. Run calibre, or perhaps better, calibre-debug -g. If you are successfully running from source, you will see a * next to the version number in the upper right of the window.
The instructions at
http://calibre-ebook.com/user_manual/develop.html worked perfectly for me. That said, another user couldn't make running from source work, so there must be some sensitivity to configuration.