Will a 99 buick century 3.1 engine fit into 98 lumina?

No, a 1999 Buick Century 3.1L engine will not directly fit into a 1998 Chevrolet Lumina without significant modifications. While both cars are GM products and use similar platforms, there will be differences in mounting points, wiring harnesses, and potentially even the transmission bellhousing that would prevent a direct swap. You'd be looking at a very extensive and likely impractical project.