The STL’s are linked in this conversation:
I’m not certain on communication with the Melzi, but that isn’t a custom board so it should have its own documentation
There are a multitude of conversations in this Google group about reviving older Athena’s which may be useful to you. As far as firmware goes, unless I miss my mark, octoprint is an alternative firmware to Franklin that you can use. The Athena’s hardware can be interchanged with any variety of of control strategies, Franklin is just the firmware of choice on wthenas because the two were developed together. Octoprint would likely provide more rich support and documentation, but may lose something in terms of calibration capabilities.
Best of luck!