I've built quite a few recording studios and am fairly familiar with sound proofing. The idea is to absorb and prevent the transmission
of the sound waves. Carpeting the engine enclosure will give a small amount of dampening but in my opinion not enough to balance the negatives of how difficult it would be to clean.
Reflectix on the inside of an enclosure will help. It is a reflective surface that would reflect the waves away from the wall into the enclosure. The air cells in it would reduce transmission
to the walls.
To really dampen it you could create a second enclosure inside the exterior walls, isolated by rubber, with absorptive material between the layers and a reflective surface in the engine room. My engine is beneath the sole of the pilot house so I could, presuming there was enough room, apply rubber strips to the bottom of the sole with reflectix or glass wool between the strips and screw something like phenolic sheet over it. The vibrations are then isolated by the rubber and absorbed by the insulation
To use recycled materials choose material that seems to fit to the principle.