Croteam has published on Github the source code of the Serious Engine v1.10, which has been used in the first Serious Sam released in 2001…
Don’t expect to see latest versions of Direct3D or OpenGL in this Serious Sam engine. The Serious Engine v1.10 has two renderers: one based on Direct3D 8 and the second probably based on a very old OpenGL 1.5…
The Serious Engine v1.10 is licensed under the GNU GPL v2.
Source code download and more information are available HERE.