Open source game engine version developed by Croteam for Serious Sam Classic (Windows, Linux, FreeBSD, OpenBSD, NetBSD, macOS, Raspberry Pi OS). Based on https://github.com/Croteam-official/Serious-Engine and linux port https://github.com/icculus/Serious-Engine
Why do you think that https://github.com/Warzone2100/warzone2100 is a good alternative to SeriousSamClassic