Just to clarify. Are you asking about the anode that actually mounts onto the engine? Or about the anode that mounts to the prop shaft?
Firstly, a disclaimer. I am not an expert on this topic.
If the engine does require a anode in lake water, it is probably a large diameter pencil zinc that screws into the mounting plug. This will be a generic item. Not necessarily Yanmar specific.
As this is a "every engine", question, rather than "Yanmar specific", maybe to go to the local ski boat engine repair maintenance shop and ask their opinion. Or call up these folks:
https://www.boatzincs.com/engine-magnesium.html
Marine grade aluminum anodes are also approved for fresh (lake) water. Likely likely would be able to locate one.
Lastly if your 1GM engine is anti-freeze cooled, rather than raw lake water, an engine anode definitely is not necessary.