'Orbiter' is a free space flight simulator created by Martin Schweiger(http://orbit.medphys.ucl.ac.uk/). 

Many people have created add-ons from NASA craft to Sci-fi models including several from Space:1999.

I have made this short video to demonstrate it's appeal.

Download the program from OrbitHanger.com from the Orbiter link and use the search link for the Space:1999 add ons (use search such as '1999' 'alpha' 'eagle' 'planet' etc.

Enjoy

Paul Stankevitch