A few game publishers in the 90's released their source code, so if you are
decent at hacking together code or using their level/modding tools, you can
just use one of the old engines.
Here's a remake of Descent using the original source:
http://www.dxx-rebirth.com/
Here are a bunch of different versions of Doom:
http://en.wikipedia.org/wiki/Prboom
Here's a review of the Quake II source code, with some cool video of John
Carmack's rather insane Intergraph workstation in the 90's:
http://fabiensanglard.net/quake2/index.php
Here's a whole bunch of games based on old open source game engines,
including the gtkRadiant level editor and quake 2:
http://icculus.org/