Well Mr. Rain... you are in luck! The surrounding areas...
appear to be very solid and some flat sheet metal can easily be massaged to fit.
You can do this kinda RIGHT by cutting out all the weak areas in as flat and straight a line as possible... then trace the outline on paper to use as a template for the new sheet metal. (Old panels from a wrecking yard could be had for a bag of sand...) Then TIG weld in spots alternating sides and keeping things relatively cool and warpage to a minimum. MIG, Gas, FCAW, or stick welding can also be done... just go about it slowly and alternate sides.
Then you can hammer, grind, sand, primer, paint, undercoat, and cover with padding and carpeting.
The other kinda right way would be to replace the entire pan.
The other kinda way would be to cut it out and then overlap the new sheet metal and weld or Rivet or screw it in place. Use lots of undercoating.
The other kinda way would be to just place a piece of tin over the top, not cutting out anything... then using welds, rivets, screws or CONTRACTOR'S Glue... secure it. On the underside... a few splattered patches of more glue applied to the rough edges... and then LOTS of undercoating or Bed Liner.
Once done using ANY of these methods, they will be structurally sound, outlast your lifetime, won't rot your teeth, and NOBODY but you hasta know!