If I'm not mistaken, isn't the Quake 2 engine just a modified Quake 1 engine? I do believe there are some references to Quake 2 in the Quake 1 source code release. Also, doesn't the Half-Life models have remnants of the Quake engine? That is other than the filename extension for the models (*.mdl), such as the use of *.qc files for defining at least their bounding box.