I’ve found quite a few of the original pumps used on the early sixties 5.5 and 10 horse motors are quite durable and still work well. But like anything else, they can get tired with age and will need replacing sooner or later. Rebuilding them isn’t an option anymore as I believe the rebuild kits are unavailable now.
Aftermarket square pumps as used on the later 6 and 9.9 horse motors are readily available and will work just fine on your ten horse. Search for Sierra 18-7350 and you should gets lots of results from Ebay, Amazon etc. One advantage of the square pumps is the inlets and outlets can be clocked in amy position to suit your needs. Doing it requires the instruction sheet so everything goes back together orrectly.
When the square pumps get old, the diaphragm inside gets really hard and brittle. Symptons are similar to yours, the motor will keep stalling at low speeds due to lack of fuel but might run just fine at high speeds.
