Got a mig welder??
Cut out the old, weld in the new. You can buy a pre-fab replacement...
Got a mig welder??
Cut out the old, weld in the new. You can buy a pre-fab replacement floor panel or make one up, depends on your requirements as far a finished product goes.
If the car is not worth much and your on a tight budget, buy a piece of sheet metal, bang it and cut it to suit and weld it in. I would go for something like 12ga sheet metal, it will then outlast the rest of the car, provided you rust proof it.