The Webway remains the same as it does to Human Explorers. the Innards of a Tyranid Spaceship I'm leaving purely up to the discretion of the GM. There's varying pictures of it all over the internet, so for the sake of consistency I'm leaving it up to the imagination. If it helps picture it, I've always seen them very much like what occurs when the Xenomorphs in the Aliens franchise begin to take over an area.
In regards to purchasing them, utilize the same stats and rules presented in the Rogue Trader Core Rulebook and Battlefleet Koronus, with whatever modifiers seem appropriate. All the basic components would fall under "Xeno-Tech or components costing more than 2 SP" as detailed in Battlefleet Koronus. Generally finding them will be incredibly difficult on the market, though possibly might be located on a borderline heretical Forge World from curious Magos Biologi. They also make for great adventure hooks/rewards since they're very hard to find.
Update: The Necron Character Guide has now been updated, and now has Necron Starship Hulls! Feedback is always appreciated, and a summation of edits is included below.
Minor graphical changes and cleanups, additional graphics added
Redid Necron Starships Section
Added Necron Starship Hulls Section
Necron Operator re-named to Harvestmaster
Edited by LodgeBlackman99, 19 August 2014 - 01:55 AM.