Well, if you haven't figured it out by now, here is my guess. The piece
that fell out is the drive pin for the water pump impeller. See if the
rounded part fits into the driveshaft about where the impeller sits.

Hope it is this easy.
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When I removed the pump housing, there was a small (1/2" long, 1/8"
diameter) round, bullet shaped piece of metal laying inside.

Does anyone know what this piece is, and where it fits ?

better yet, does anyone have a shopmanual page for the pump assembly,
they can share or trade..? (trade for 1980's Honda BF7.5 4 stroke
shop manual)