There's been a few I've googled that basically was an Imperial Knight, using the unfished skull face. The main gun was the one from a... Doomsday Ark. I think that's the kit. It's a huge gun, perfect size for an Imperial Knight's arm.
If I had to stat out a Necron Knight, I'd probably make it standard armor 13/12/12 and give it Quantum Shielding, instead of the Ion Shield. Some massive Tesla or Gauss cannon, or maybe even the Death Ray. Shoulder mounted Gauss Blaster or Telsa Blaster (same optoins as an Immortal). And replace the Destroyer Chainblade with a special Destroyer Warscythe.