I'm pretty sure you can also just build enough blocks around the area to allow them to spawn again.Is it about the Cultists not spawning? You can check if you still have a valid marker for the dungeon with TEdit (I think it works with 1.4 now, not sure though). If necessary you can place a new one.
It seems that the Dungeon marker can become invalid (position reset to 0,0 or something like that) if the dungeon entrance is destroyed too much. You can fix that with TEdit.I'm pretty sure you can also just build enough blocks around the area to allow them to spawn again.
Thanks everyone for the help I built some more around the dungeon and the cultists respawned!! Again, Thanks.