Hello fans and devs of UFO:Alien Invasion !!
I propose that UFO:AI changes its engine from Quake 2 to Xash 3D since its more advanced and its compatible with Quake 1(Maybe Quake 2 - IDK) and GoldSource engine.
It has advanced features and graphics comparable to Source engine, It even supports some simple physics. I saw it on some youtube video, I wish you guys contact Xash 3D dev team and I think they will gladly help you since they are modders...
http://www.moddb.com/company/xash-xt-groupList of features (from a kinda outdated build, there is an post about update...)
http://www.moddb.com/engines/xash3d-engine/features/xash3d-engine-v095-features(about build 2153 = latest build since near end of 2012)
http://www.moddb.com/engines/xash3d-engine/news/xash3d-build-2153-xashxt-v061-are-releasedI think it can be compatible with your AI and scripts mostly. The conversion could be rather simple and may slightly slower development of UFO:AI if you decide to convert it but it may result in smoother development since graphically you wont be limited.
Since it what it can do, I think its well optimized. Think about it.