What motor brand year and hp? It will be mandatory to pull the thermostats and poppet assembly to clean them and back flush the cooling system. Pull the hose off of the pizzer at the block and cowling to open it up. Try to borrow a temp gun so you can check the motor for hot spots. I have had to remove cylinder heads, exhaust cover and power head before to clean the trash out so motor would cool. Had a Merc V6 that sucked the exhaust adaptor full of small gravel.
“The world will not be destroyed by those who do evil, but by those who watch them without doing anything” GOD IS GOOD