The pump is a Jabsco 2760-0001.
I replaced the gasket but no joy, it still leaks.
The rebuild kit costs more than a new pump (go figure).
With a 29 year old engine this is probably just the tip of the iceberg.
I am thinking of replacing the engine with a new Yanmar 2YM15.
Hell, if the new engine lasts 29 years I'll be 94 before I need to worry about another replacement.