You should have found all the
You should have found all the impeller parts from the old pump- if not they really loved to go wedge in the most inaccessible places.
My first look-see would be to remove the thermostat. Next would pull the powerhead and look where the water jacket meets the cooper water line from the pump. You also get a free look down the hole to the water pump and see if the tube is in the pump correctly. After that the exhaust cover is the next place the parts liked to wedge. Then the head gasket and water jacket. Finally- where the water dumps into the exhaust.
All of these places can cause your problem.