Harvester upgrade sprue or STLs would seem to be a no brainer.
But also, any 'Nid player is likely to have, or be able to trade for, any amount of gun arms. Tyranid Warriors (which these seem to be the right size for) come with a wide variety of options, let alone the other model types.
(EDIT: On further look at the sample shots. they appear to be more like large human sized. Somewhere between a 'Gaunt and a Warrior. Those big legs do add a lot of apparent size! Termagaunt guns are probably going to work.)
For skirmish size use (i.e. Stargrave, RPGs), these are as likely to be used as alien animals (no guns) or to be converted to carry tech equipment (chop off the scythe bit and glue on a hand with weapon, or even just glue on an arm). And if Biowarriors are desired, that still gives ten gun bugs per box.
The bodies would probably mix well with GW Ork or Kroot limbs and weapons if you want brutal looking techno-crabs. Converting the legs to have claws instead of scythes and an upright posture should also be fairly simple.