I remember reading somewhere that the repair pieces you can get for Series doors can be used. By carefully removing some of the outer alumimium skin (i.e. peel back some of the folded over section), and placing a protective piece of metal between the frame and the skin, you should be able to cut away the rusty section and weld in some of the repair pieces, once painted you can remove the protection piece and gently fold the alumimium skin back into place.
In theory anyway.....
The other option would be, if you the series repair pieces don't fit, is to follow the same process and cut out the rusty bits, and use these as a template to make your own repair pieces.
I must admit, this is just theory - never tried it myself.
Ian