Looks like there is also another large piece missing under there, which covers part of that elongated whole (slot) and runs toward the rocker area. Unless you are qualified and have the equipment, I would suggest a frame shop, like a shop that builds and repairs truck frames. It doesn't look like that serious a problem, I certainly wouldn't replace the frame, anything can be repaired and it will give you a good reason to clean up everything else on the bottom side!