When they kept saying "only the engines are Forerunner" in SpOps, it sounded to me like they were implying the engines were 100% Forerunner, and they just had to figure out how to hook them up to the ship. It's never really been specified. Although the light in the engine area uses the same texture as the hard light in most other areas of the game. And there's those floating bits off to either side near the engines that don't physically attach to the main hull.

I just kind of assume that Infinity is a treasure trove of Forerunner tech plot devices that they can use later on if the story demands.