This is the head of my 70 hp yammie.
The motor was overheating at low revs,ie when trolling livies and also at WOT, but not in between.
When I looked at the thermostat it looked a lot like yours.
I always flush the motor on earmuffs but evidently not enough.
The tap water being cold, limited time and being conscious of driving my neigbours made I probably never gave the engine time to get hot enough to open the thermostat for long.
Even after cleaning it up the stat opens very little at idle.
Pressure from the hose is bugger all when you look at the "relief"it gets through the earmuffs, excess pressure just spurts out the sides.
A Drum would allow for the water to warm up, meaning more flow through the t stat.
It would also make a reasonable silencer so you may be happier to run it longer.
If your impeller is missing parts you have wonder where those parts are now?