you dont neccisarily have to have the source code to port something, all you need to do it produce code that reproduces the functionality of the original.
if you made a game engine that could use all the same data files etc. as the original tomb raider it would effectively be a port. and potentially a legal one. some programmers made an executale for unreal tournament that ran on linux and used all the original data files that worked so well the game publishers actually recommended it for linux users, of course you had to buy the game to get the data files unless you only played mods
