A few, but none of them any good. Unfortunately I didn't find time to dig out an original cap, so I'm doing this from memory. But as I recall, there is a post that sticks down into the opening when the cap is closed, meaning that you couldn't put a lock in the top of the opening. And to put a lock down lower is going to require something to prevent it falling into the tank.
Wonder how the size of the original opening compares with a modern screw-in cap ? Perhaps you could fabricate threads (or modify a filler neck from a modern car) to fit inside the original opening. Then you'd have to cut down a matching locking cap, but that should be easy.
Another, more radical approach might be to blank off the filler neck from the original cap (leaving it in place for appearance, but useless) and fill the tank from inside the trunk. Probably lose some tank capacity (and trunk space) that way, though.
A sleeve to fit inside the opening, with holes for a custom cap to engage is about the best solution I see, but lots of fabrication there.