I had a quick ponder, then compared actual models, I'd say the Tau XV25 stealth suit's head/helmet thing is quite close in size and shape. Could trim the bit that goes over the head and round it off with greenstuff. I compared it to a scout torso, rather than powerarmor, and they'd fit well. Could tinker with the vision optics, possibly even fill the hole with
GS and just make a narrow vision slit. Depends how close you want to be in shape, the more accurate option would be as you say just
GS the head.
The marine assault squad box has a outstretched (pointing) powerfist arm, should be easy enough to readjust the finger so it's not pointing. Then could have a scout (or even
IG) arm opposite, seeing as the concept art only has one armored arm.
Would be a neat conversion, be sure to show it off once it's done.
Edit: Forgot to add the XV25 picture..