Ok how hard is it to repair sprockets ,or are there some to replace them with

Depends on what you mean by broken......if it's spokes that are cracked they can be welded up if within reason. If it's the sprocket ring, new rings can be fitted. If it's hubs ya need a whole replacement, good used would be fine or can be repaired as described. The preferred method of replacing rings is to pull the sprockets to do the work. It is also possible to cut the ring in half and weld it in place without splitting the track and pulling the sprocket but controlling alignment is more difficult with this method.

some of the spokes are cracked and welded ,I know today the welding on cast-iron is a little better

The sprockets are high grade cast steel, welding is not a problem if done with reasonable care.