It is fairly easy to over-plate the corrosion using MIG or TIG welding.
If you can fold two half sections accurately to suit the outside dimensions of the box section, with the joint lines on the fore and aft centreline. cut out a slot to fit around the centre stand mount and weld to sound metal.
Normal arc welding will be difficult as you will tend to burn through but using MIG on a thin wire (1mm or so) Not the flux-cored crap but proper gas-shielded MIG. At a pinch, you can gas weld but MIG is easier.
You will have to clean any paint or rust as MIG doesn’t like a dirty joint.
Most garages or welding shops will do the welding if you make the parts.
The slot around the centre stand tube have to close enough to allow welding of the tube to the new plate.
I did one a few years back and it worked perfectly.
The other option is to buy a scrap frame and cut the bottom portion off and butt-weld it to your frame.
“Cut your way in and weld your way out”