I've been replacing annually (mostly) just the impeller, but realized the o-ring was flattened and a bit brittle. West Marine offers the Johnson 812 kit that includes impeller, o-ring and a gasket ($39.99). WM P/N 4608162
Gasket doesn't fit, but M1UZ manual shows no gasket, so we're cool.
Anyhoo, learned from overhauling my 1995 Toyota 22RE engine that o-rings go bad with time.