A studio like WETA would usually be building something from the ground up - modelling, rigging, animating, compositing, etc.
Considering what can be done with consumer-level tools today like After Effects, the compositing needs of Alien3 are actually fairly simply (no handheld shots, nice clean plates, etc.). We have better, faster digital tools at our disposal than the photochemical or rudimentary digital processes available back in 1992. There are people on this forum -- myself included -- who could do a better job of the compositing work with a subscription to Adobe Creative Cloud than what people saw in the cinemas. This isn't to brag, just to highlight how far tech's come in the last 27 years (that being said -- if the elements were shot with too much spill from the screen, you'd absolutely need a professional studio to fix it up in anything resembling good time.)
But if we're going to turn it into cleaning up existing shitty mattes, that turns into a whoooole other set of problems. Primarily you're introducing a lot of hands-on, frame-by-frame work you can get around when you're making the composites from scratch. You've got to remove colour casts and matte lines that are baked into the image. That will involve rotoscoping, secondary colour corrections, background replacements, and a whole slew of other fiddly things (especially in the shots where the Alien's shadow has been animated to react to flares being thrown past it -- shit!).
Possible, absolutely -- and, hell, if you had the money to throw at people, you could still get dedicated amateurs or semi-pros to do frame-by-frame touch-up and get pretty damn good results -- but it's a loooooot of effort.