I have not really found a site that lists the engine and tells you the impeller.
I did find some cool sites that tell you how to change it and things to look out for.
And wikipedia told me the F means fresh water cooled http://en.wikipedia.org/wiki/Yanmar_2GM20
How to change it...
http://www.ehow.com/how_6017596_change-impeller-yanmar-diesel.html
It appears I want YAN124223-42092, but I would like to verify that part number somehow.