If you're good at sculpting, just GS it. If not, fill the cavity with GS, then ALL THE PURITY SEALS!!! It's what they're for, pretty much.I'd be most concerned about the gap on his chest on one of the wings (where one of the nid's claws fits). You'd have to fill that and detail it somehow.
'My, this model looks amazing apart from this unsightly gap. I could mix up some filler and make it AH STUFF IT, PURITY SEALS!!!'