Extreme Tux Racer is an open source racing game featuring Tux the Linux Penguin. ETRacer continues in the tracks of Tux Racer and its forks. ETRacer is based on the PPracer code (which in turn is a fork of TuxRacer) and is a continuation of these two projects, since they're both rather dead. There is also already an ebuild present on their download page: http://www.extremetuxracer.com/?download
I tried the ebuild (for the latest release 0.35) from the official homepage of Extreme TuxRacer, added "~amd64" to the keywords and succesfully emerged it. I also tried to play it and it seems to run ok on amd64.
Package is now in portage. Thanks