this guy was able to make a silicon chip with 1200 transistors on it in a garage, far from a modern CPU, but spitting distance from the 3500 transistors of the intel 8008 CPU. Projects in flight also has a series working on DIY semi conductor fabrication he’s not quite to the stage of fabricating a full chip, but he’s covered a lot of the more difficult parts already. And this is all stuff being done as hobby projects.

Something like an early pentium chip like the P5 would not be a particularly difficult task for a well resourced team. The questions are how much could they rely on the currently available off the shelf supplies and tools and how much do they need to scale production.