I'd carefully pull the push pins and remove the whole thing, wrap the removed 'sink in some bubble plastic and put it on a shelf, for RMA purposes. You wouldn't get anywhere without it.
Then cut up an old heatsink from the junk drawer to replace the portion you arrowed. With holes through the mother board, it would be cake to mount a home made replacement.
I would include a 'sink for those grey cubes while at it too, but that's just me.
Use Krylon Barbecue flat black paint to make the home made ones follow the color scheme, it is a highly heat transmissive paint.