Aliens - This Time Its War
Author Pete Jones has done a completely new version of the the rules to play Aliens by adapting the rules of the old Milton Bradley boardgame "Space Crusade" These new rules utilize dice rolling for all alien appearances and actions. Instead of using pre-done maps, the game is played using tiles which are randomly placed as the game is played making the game unique each time (similar to games like Space Hulk or Dungeon Quest).
The game is freely available as a downloadable PDF from the Freewargamesrules website.
The direct download of the PDF is here: http://www.freewargamesrules.co.uk/uploads/7/0/8/1/7081303/aliens.pdf
While this is a completely different set of rules, the games share a lot of similar qualities and tone, and so it is quite possible to mix and match rules to create your own hybrid "house rules" quite easily.