I've recently been in contact with Duncan Brown, the guy who wrote Alien Arena, which was recently added to MAME. Alien Arena was an entirely new game (in 1985) that was designed to run on the classic Williams hardware. Duncan owned an arcade at that time, and he actually put the game on location there with the hopes of making it into a more widely distributed conversion kit for arcade operators.
Duncan has been kind enough to allow us to make his game available free for non-commercial use, just like the other games in the ROM Images section. So I invite you to check it out, read more about the history of the game, and enjoy playing something new. Duncan has even provided his email address in the enclosed readme, so feel free to drop him and line and let him know what you thought about it!