Bruce Bailey here on SOTW forum has added some front F keys to old TT horns, if my memory serves. Look through the Buescher forum and you will find many posts by him. Send him a PM, or a email. Sometimes he posts a phone number where he can be called.
There are many sax restorers who make custom key fabrication as well. Try TENORMADNESS and WORLDWIDE SAX for starters, as both have large web sites. Also try a Google search for Sax Custom Keys, or some variation thereof.
Apologies to Bruce for suggesting him if he is not interested.