Plow: if you can't find a bottom, dad would use a plow lay, and weld it into place there across the bottom to make the old one work longer. Not a perfect solution, but if you cant find one any more.....
Disk: I guess you weren't angling it? Oops.
As well, it is worn out - the blades are worn down too small. It'll work, but there isn't much bite left in it any more. New blades will cost a bundle, but will make a good disk out of it again.
Weight is good, helps it bite into difficult ground, esp with the blades worn out like they are.
Trouble is the very wet mud you are showing on th wheels and disk itself - disk doesn't work worth a dern in muddy conditions, only packs the ground down to concreete.
--->Paul