The source code is available via Github. Bug-tracking and feature requests are also handled on the Github project page. For an overview of setting up your own Tworld server, see the Installation page.
Tworld does not contain any references to Myst. The code refers to “portals” rather than “linking books”, “worlds” rather than “Ages”, and so on. All of the Myst flavor in Seltani is a customization of that server. (This customization comes from editing the HTML templates and the localization table of the Seltani database.)