I am seeing that is very similar to Alien system: Characters have more talents (they are different), stress looks a bit different and there are different skills but, in general, it can be adapted to Alien rpg.
The background, the characters, events, the maps and the story are really awesome! There are also agendas for every scene. There are five scenes.
The only objection, in my opinion, is the assimilation process (infection) the Thing does to a human. If I am not wrong, he/she has to touch the player and yell an interjection. It's not said that he/she has to do secretly (I understand, since it's not told, that the Thing does it in public). That fact would reveal the Thing right away to the players. I insist that maybe I understood it wrong but if not, I would use another system: for example, through secret notes to the GM when players are alone with other characters/players (they have to pass a note even if they do nothing, thus increasing the paranoia and the secrecy) and keeping a tight track of every action in different locations.
Other problem is that players who have seen the movie could act according to their minds (not to the characters minds). For example, staying always together so it would be impossible for the Thing to assimilate another character. I think this is solved with the "agendas" proposed to the characters and the items they have (keys to some sections, for example). Some of the agendas could be a bit unnatural: for example, some of them tell (in other words) "from now on, you distrust X character"... Maybe the flow of the game session has gone in other way and that's the reason I say that could be a bit unnatural.