Is a 1854 San Francisco gold 20 coin worth anything if it has copy stamped on the back?

COPY should be a dead giveaway. The coin is a replica, not the real thing. It's very unlikely that it contains any gold. or maybe just a thin coating on the outside. A jeweler or coin dealer could run a specific-gravity test to determine if it could potentially be made of precious metal but nearly all replica coins are made of base metal.