Alright, I played with it and changes just minor things, and ended up with a version that works well for me.
http://files.timbru.com/alfred/Search Keychain.alfredworkflow
Some changes:
Works on Big Sur
Keyword ka without a parameter just launches Keychain Access
Keyword ka with a parameter will launch Keychain Access and search for the keyword
Using the Cmd+Shift modifier will copy the first search result, e.g. similar to the Keychain Access built-in behaviour
Using the Cmd modifier will launch Keychain Access. Now that I type this, not sure if needed, I'll probably remove it.
I hope it's useful to others.
Have a nice day.