If you can get a very good preferably black and white photograph of the original decal taken directly over the centre point of the decal in order to avoid any problems with perspective, then there would be no reason why a silk screen printer could not copy the decal exactly, and print it out onto vinyl at whatever size you want in the colour/s you want. But they would have to make up the silk screen using a printers camera, and therein is where the costs tend to run away, if you only want one decal, then that is the most expensive option when you could have another fifty printed out for very little more money. Having said that, things might have changed in the last thirty or forty years with digitisation to enable those one offs to be done at more competitive prices.
