Sometimes the seal and grommet at the base of the powerhead gets swollen or even melts if the engine suffered an over-heat.  Of course the blockage could be further up in the block but I would check the base  area first.  Powerhead has to come off to do that.

I had a KF5 that acted the same way. Took off the waterjacket from the cylinder & exhaust/water jacket & stuck a piece of wire, long screwdriver & an ice pick in  the water passage holes & sprayed carb cleaner in the water passages, then blew air & water thru & then had water flow thru everything backwards & forwards. Never saw a big chunk of anything but it ran very cool afterwards. If I may suggest check your gearcase drains too if byou havefreezing winters.