Harjula pointed out a new source release for an existing commercial game, Enemy Engaged: RAH-66 Comanche Versus Ka-52 Hokum by Razorworks. Although I’ve never heard of this game, I have one data point indicating that is was a pretty decent game. You’ll find a link to the source code along with other resources at the download section of the page. The source release at least makes possible a Linux port by a flock of dedicated Linux combat sim fans.
Note that you should read the LICENSE.TXT file before throwing yourself into the code. My quick read over the license, which Marv extracted for me from the Windows-centric zip/exe package, makes it sound like you’re free to modify the source as long as you (a) do not distribute improvements without redistributing their license with it (b) do not profit, or allow anyone else to profit, from the source code, binaries, or improvements made to either, and (c) give any and all improvements back to Razorworks. I realize that most of you come to LinuxGames reguarly to obtain legal advice, and I thank you for that, but I’d like to remind you that IANAL*; read the license yourself and then decide if you want to play by their rules.
Without a doubt, the original game is required for doing most anything useful with this, but it still appears to be for sale online. Perhaps check your local game store bargain bins before purchasing online.
* IANAL means “I am not a lawyer”. That’s dirty enough. Don’t make it any worse.