Broken cast iron can be joined together by a skilled craftsman, if you can find one.
If all the broken pieces are available, it may be possible to glue them together with a specialist adhesive, perhaps with a thin backing plate to give stability.
Missing metal plate/milepost
If you have the broken pieces (or an equivalent milestone) these can be used to make a pattern and cast a replacement plate or marker. There are foundries that will undertake this work. The cost may be hundreds of pounds for a flat plate, around two thousand for a cast iron triangular section milepost. The necessary pattern may cost more than the casting. When fixing mile-plates on a backing stone, you should use self-locking bolts to prevent the plate being levered off and stolen. You might also use specialised adhesives behind the plate for additional security. There may be an electrolytic interaction between some metal bolts and plates.