i dont think its quite as simple as just replacing the intake manifold.
It seems that the rods and pistons are different and that it has higher compression just from a couple things ive been able to find. (theres not much out there on it, so im not sure if its legit or not)
I have a feeling the ecu is different as well since its meant to use natural gas and probably shoots in a different amount of fuel into the engine.

The best way to find out for sure tho is probably to try finding the service manuals on both the gx and ex civics and compare.
Sorry i cant be of much help, ive never heard much on the civic gx
Im pretty bored right now tho, so ill see if i can find anything that might help