I think......no promises here....
You need to remove/unbolt the hanger the rear spring is attached to. Then make good the pocket and surrounding floor pan.
I understand it is one of the more difficult jobs on the midget as you need to relocate the suspension mount accurately as to not mess things up.
These guys are really helpful http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=tlist&subject=97
It doesn't look too bad, But it will look worse when you remove all that underseal.
If you can get a repair panel, do so but just use what you actually need from the panel, cutting only the rusty metal from the car, and shaping the repair panel to fit the resulting hole.
If it will mean disturbing the spring hanger, make a jig after removing the underseal but before you cut anything away, that will enable you put the hanger back in exactly the same position. I have done this more than once on landrover chassis without issue. Also take many, many measurements and photos, It's better to have more information than you need. Handy also if you can get a dimensional drawing for the shell.