Have you seen Long Mountain Outfitter's website? LMO bought out many of the movie guns from Stembridge Gun Rental several years ago. Here's a link to the Aliens II piece...
http://www.longmountain.com/movieguns/Aliens2
As to your question about building one, you could build a semi auto one, but not a full auto. If you wanted a full auto version you'd have to start by buying a registered full auto Thompson (which will cost you a bare minimum of about $13,000-$15,000 on today's market for even a low-end Auto-Ordnance gun) and adding the hollywood "makeup" to make it look like the Aliens gun.