8hp yamaha impeller problem

[twalker8204]

hey everyone new to the forum here and this is my first post. I had to replace the impeller on my '89 8hp yamaha long shaft outboard and when i took the impeller housing off the impeller had no blades on it. I recovered most of the blades but there is one piece missing and i have a feeling it is blocking the water from coming out of the pisser. i also decided to replace the thermostat since the plastic collar melted around it. The motor was not run too long but apparently it was hot enough to melt that.
So does anyone know how to flush the cooling lines out or what i should do? Is there anything else i should check?

Thank you in advance