Can a 2001 ford 4.0 engine fit in 1991 Ford Ranger?

No, a 2001 Ford 4.0L engine will not directly fit into a 1991 Ford Ranger. While both are 4.0L engines, there are significant differences in the engine's mounting points, accessories (like alternator and power steering pump placement), and potentially the bellhousing (connecting the engine to the transmission). You would need extensive modifications, which would likely be more expensive and complicated than just finding a suitable 1991-era 4.0L engine.