Have you tried Beyond Earth? I know it's not _exactly_ the same thing, but from someone that's also an Alpha Centauri fan, I can tell you that I have greatly enjoyed Beyond Earth. You can get it for 15 dollars (with the expansion, too!) pretty regularly if you set a watch on isthereanydeal

Consider that if you have more than one screen/display then activities are a significant difference from having multiple desktops. Also, the ability to pause and restart an activity is helpful. I have anywhere between 2-5 activities with two displays and two desktops each activity, so for me, I would be swimming in desktops if I didn't have activities to keep things organized.