Someone posted a link to a chart that showed the engine modifications by serial number over the years. It was on this board, but I don't remember where I saw it, sorry. It was in the Tech Forums, I believe. In general, I think the K bikes saw decreases in horsepower through the years, and the F had some changes in gearing. The gradual loss of power may have had more to do with emissions control and carburetion changes than the engine. Thoughts of Hondaman in the FAQ section has a lot of good information about the K bikes but not much regarding the F engines.
There are frequent comments about premature valve guide wear in the 77 and 78 F engines but the guides can be replaced with beefier versions.
Disclaimer: I have never actually been inside one of these engines, unless you count adjusting valves and replacing sparkplugs. Just going by what I've read here.