Why Audacious machinery could not simply move into Hood

Could HMS Hood's reconstruction have used the power plant from an Audacious-class carrier hull?

No. Once installed, the machinery was too deeply integrated into an advanced carrier hull to be a practical donor.

An Audacious-class carrier power plant could not simply be moved into HMS Hood. By the time machinery was installed inside a hull, the new ship was already well advanced and approaching launch, with major surrounding structure in place. Removing that plant would disrupt one nearly completed warship to support another. Hood might receive newly manufactured machinery in a reconstruction, but an integrated carrier installation was not a practical donor.