The Unreal Engine 2.x, at least the rendering portion of it, was ripped out. The replacement was called Flesh, and IIRC the guy that implemented it was probably an external contractor who did his job and left, but noone asked him to leave any documentation on it, so he didn't. They also used the first implementation of Havok for physics, literally like version 1.0, which was buggy as hell, and they couldn't get water and rope arrows to work.
Memory constraints were an original Xbox thing, but it's hard to tell how PCs from that era would handle bigger levels or no loading zones.