I think you are asking about putting in a plywood table in the opening where the existing iron table is located. The wood table should fill the entire space, so it would be larger.
I suggest you build a table that fits into the space, but is half an inch short, using adjustable bolts in the bottom of each leg to level the table with the saw. Then you should be able to drill and add some screws or bolts. Drill throught he angle irons and into the table to secure it once leveled.
I'd make the top double thickness. Lay the plate where you want it, mark it and cut a hole half an inch smaller than the plate through the top. Fit four pieces of wood as shown, around the plate. Add a playing card extra thickness so the plate is easier to remove. Use double sided tape to hold the boards down, then use a mortising bit with a top bearing to route out a rabbet, using the boards as a pattern.
I would enclose the router with a hinged door to manage some of the sawdust. Stick suggests attaching some tubing to the bottom of the router that draws in filtered air through the router for cooling and to reduce sawdust incursion.
I would add a nice drawer underneath for storage, but also to provide a stiff framework for the lower part of the table. It would add rigidity.
Kreg makes some leveling screw devices that go in the corner of the plate opening so you can level the plate. They are really nice. You can see in the picture that you don't need a rabbet for the plate, but it will prevent the plate and router from falling into the opening. The diagram shows the rabbet, which is slightly deeper than the plate thickness.
If you want to get fancy, you could add T track to the sides, front and fence. T track gives you lots of extra options. I would also make a really nice tall, split fence. Several designs exist for such a glorious fence.