It really depends on how the original work surface is connected to the frame. If it’s screwed in or held with nuts and bolts then you could replace it with a piece of identical sized but thicker Plywood or even a different material. The best bet though is to buy a workbench already equipped with the type of surface that you want, and if you don’t think that Plywood will be up to the job, consider picking up a table with a hard wood surface.