Hi - a fairly bullet proof motor
Back when I had a 15hp Yamaha 2 stroke
it was common for water
not to flow out the ?indicator jet at the back of the motor
due to salt/corrosion blocking the junction of the rubber hose to the plastic jet inserted into the case below the power head
. Just remove the rubber hose from it - if there is flow then clean the jet.
Most recently had blades missing from the impellor and had to do a major disassembly to find them in the water
ways. You are lucky that the impellor is intact (but old)