I needed a jewelers loupe to find the numbers on my 1929 rumble seat latch, but it was on the square shaft under the rust & grime. Finding that number would be the easiest as there are a lot of people around that can cut from the number. If not, I'd take it to the oldest locksmith you can find locally - the one in Rockland was able to key a model T fat-mans steering wheel for me, it was not as cheap as having a key cut by the numbers but able to be done. For comparison, the wheel was around $40 for 2 keys, and having them cut by the number was closer to $10. You will pay more for a reproduction original style key blank too.
Good luck