I have an Old Coney Island, which is one of the few titles that Game Plan made that was full-size (their forte was cocktail table games). The one I have is missing the lock for the backbox, although the rest of that assembly is intact. The cabinet seems to be a clone of Bally of that era (late '70s), so one would presume that the solution for a Bally like this should work here - but I'm not so sure.
Hopefully, someone here has a Game Plan title with this problem and has found a solution.