Hi Mark. If the motor has been sitting for a long time, the water pump should be one of the first things to be serviced before running it. The OMC/BRP part number for the very common original impeller is 434424 (Sierra 18-3001) and is readily available. If you decide to replace the whole pump, the updated pump kit with housing is P/N 763758.
The correct base gasket for your motor is P/N 303467 and if you were to use a later ’56 – ’79 304314 or 313065 base gasket, the motor would have a massive water leak under the powerhead so the correct gasket must be used.
Once the gearcase is off, take a look up inside the exhaust housing where the copper water tube from the pump enters the exhaust housing. There is a rubber grommet up there, same as the one on the pump, and the aluminum around it can corrode away causing a cooling water leak. This is more of a problem for motors which have been operated in salt water.
Just a reminder, but your motor runs on 16:1 gas-oil mix. Not 24:1 and definitely not 50:1. Doing a Google search for the part numbers will help to locate parts, either locally or from China.
Here’s what the 763758 water pump kit looks like. It has a much smaller impeller than the original one but they work just fine.
EDIT: I found a kit on Epay near you in the UK and it’s selling for about half of what I have to pay for one here in Canada. It’s an early P/N but the same kit. Here’s the link:
