There would be significant modifications to change an E to a J. At the very least is the nose and most likely the wings, slatted vs. hard, depending upon which E kit you have. Internally the cockpits had visual differences as well. Those decals look to be 1/32, so the quick answer is to buy the 1/32 Tamiya F-4J. Keep you E and build it with whatever markings the kit has, buy new markings, or dispose of th kit. Who knows, perhaps you might find someone who wants to swap their J for your E…