I think it was Frank Robb who posted the following info about this fuel pump and the earlier version. Also Ken W posted info about Nitrile gloves that can be used as diaphragm material. I’ve tried both approaches and they work fine.

Well OMC declared that pump to be unrepairable many moons ago, and quit making parts for it. But another member pointed out that you can use the diaphragm and diaphragm support from a 393103 kit (Sierra 18-7820). I tried it and he is correct. Those are the only parts in the kit that you will be able to use. Take the pump apart at the seam closest to the motor, and try not to disturb the other seam or you will be making a complicated gasket. "Move" the diaphragm’s two lower corner holes upward about 1/2 a hole. Assemble the pump, then use a razor blade to trim off everything that hangs out. Worked for me.
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There were two look-alike versions of that old pump.

One had "real" check valves with springs behind them. That kind was an excellent pump, but a PIA to assemble.

The other (improved) design had a rubber gasket that had flapper valves instead of the discs and springs. If it is, my advice is toss it and get a fuel pump. That kind wasn’t any good when it was made. My 2c.

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