Haven't taken my mine off yet to see what is in it but does anyone know if the translucent part of this would be available as a spare as mine is cracked although it doesn't seem to leak.

I think you will have to buy the whole thing. They are not terribly expensive.
I had one over my stove. Good for letting heat and smells out but not much else I fear.

I have one of those on deck over my Lazaret, removing it allows the emergency tiller to be attached to the rudder stock. The translucent part (on mine) is a solar cell that drives a little fan. I could hear the thing running when I first bought the boat but didn't know what was making the noise. Only worked it out by mistake when my head was down in the lazaret trying to find the source of the noise and I accidentally covered it up and it stopped spinning.