When I play classic Serious Sam, I use this port with Vulkan rendering. You can just grab the AppImage and drop it alongside the original executable. I believe it also fixes the infamous half fire rate bug. (I'm genuinely not sure; I usually play each level "from scratch" via the level select, so it would never be a problem for me in any case.)

However, the original game is still perfectly playable with a simple config edit for higher resolution and FOV.