You don't use an FPS engine to do open space simulation. Want an example of WHY? Go watch 3dmark2001 at the scenes where the cameras rotate up and look out from between building into open spaces (the harbor).
Enclosed space 3d engines are VERY different coding-wise than open space 3d engines. You can fake it, you could twist things out of shape with pliers and MAKE the engine do what you want, but that's silly.
Zazicle's suggestion about Iwar2 and Flux would be good but for one thing: the juicy, tasty tidbits were hardcoded. You'll have to fake a lot of stuff in ugly ways (witness the problems with making a proper cluster missile, or proper turreted beam weapon).
If I had my choice, I'd take Iwar2's Flux engine, with POG stripped out and replaced with Python, and all hardcoded bits switched to Python scripts. Further, I'd have the geometry loader understand GMax files, though LW5 is still acceptable with most cheaply available converters.
Call me greedy.