I would take your missing key problem to Irwin Marine in Laconia and run it past their parts department. Could be there were only one or two or five different keys for this old control, and they have a set of these keys that's been there for the last 70-years, or something.
Thurston's Marina, Fay's Boatyard and Riveredge Marina in Ashland are strong possibilities.
www.bentonofboston.com could make a key for it, via the mail, and it is most likely a very uncomplicated type of a key similar to a 1950-60 car key.