I don't know this particular model, but on a Bigfoot typically the tanks are held up to the floor above the basement my straps. The covers you see cracked are just covers and have no structural function. I would not attempt to make them have a structural function as there is nothing very solid to tie them to. The structure is all in the sides of the lower box and in the floor that you walk on, above the basement compartment.
Reinforcing the tank mounts would mean better straps or hanging something off the floor. The foam that you sometimes see under the tanks I do not believe is for support, it is for insulation to keep them from freezing in the winter.
Replacement covers could be made from any this, suitable material. The best would be real fiberglass panels like G10 material from McMaster or others. This will not crack nearly as easily as the chopper gun stuff used by Bigfoot (which is similar to shower enclosure stuff). Either will cut and drill with ordinary tools. You may as well drill new holes, through you could use the old cover as a template to match the new one. The screws merely attach the cover, they do not hold up the tank.