Welcome Bernadette and sorry to hear about your troubles. The floor structure is a sandwich of 2 layers of I think 6mm plywood, with polystyrene. You will need to build this up as I have not seen the "sandwich" available ready made.
I'm guessing you have somebody lined up to do the welding, who should be able to source the correct sized tubing for the repairs.
Best of luck with your efforts and well done for getting another classic Eriba back on the road and into use.
Colin