I’m pretty sure that those exhaust
manifolds don’t last long either, expect to have replace them if you buy the boat, if not they often result in buying
a new motor
I’ve had good luck with flushing
and flushing and flushing until you only get clear water, then the 50/50 mix, run a year and flush again until clear water and often that’s the end of it.
Install the Prestone flush kit in the heater
hose and flush until clean, run the motor
flushing still, then close of drains and run until hot, drain and flush, then add antifreeze
Sounds like a lot of work
but it’s not really.
When you flushing leave the radiator cap off and all drains open, and just sit back and let the bilge pump
do it’s thing.