Pretty much all the Honda bikes from at least the late 60's through the mid 70's (when they switched to the recessed filler caps) used the same gas caps and latches, so finding a cheap used one that will work for your bike shouldn't be too hard, especially if all you need is the lock/latch. Don't worry about not having a key for the "lock", since they aren't actually locks, just latches that can be opened with anything that will fit in the slot.