See:
https://en.wikipedia.org/wiki/Nokia_N900#Processorsyou would have to cross-compile a subset of FG, and would have to port the remaining parts to use OpenGL ES (according to wikipedia, the n900 uses a SGX530 GPU that only supports OpenGL ES).
Either way, 256MB of RAM (or 1gb of virtual memory with swap) will be problematic - FlightGear is not particularly good at feature scaling, and isn't designed/developed with embedded platforms in mind
Someone with a backgound in software development (Linux, C++, cmake, git, OpenGL) could probably get
a subset of FlightGear built for the device, but that will be primarily useful as a learning experience, you will not be able to actually load any scenery or complex aircraft.
It would still be a good effort though (primarily for FG itself), but certainly not for the faint-hearted.
You'd basically start out by building the headless portion of simgear/flightgear and its dependencies, and then take it from there by incrementally re-adding stuff.
But again, given the hardware specs, you won't be able to "fly".
For additional info, refer to:
http://wiki.flightgear.org/FlightGear_and_old_Hardwarehttp://wiki.flightgear.org/Howto:Optimi ... le_deviceshttp://wiki.flightgear.org/FlightGear_Headlesshttp://wiki.flightgear.org/FlightGear_and_OpenGL_EShttp://wiki.flightgear.org/Feature_Scaling