unfortunately, Megabloks does not sale decals for vintage boxes. They are impossible to find without buying a not used set !
You will find bellow a  scan of an authentic decal plate.

You may ask a professional print shop to print it on adhesive paper (be sure he will work for the correct adjusted size).
If you use a personal printer, the result could be poor.

The exact size of the decal page, black to grey / black to black, is 34,3 cm x 24 cm (do not count the white borders).