If the fin has popped out of its socket, you should be able to put it back in or plug it in with some adhesive if it's loose.. If it's snapped, you're probably going to need to glue it back into place with a plastic adhesive like Tamiya's Extra Thin Cement, or you could try to plug in the gap with some putty and sanding or just leave the base of the broken fin in place for a battle-damaged look or if you don't want to go through the effort of a custom build. You can also try to take off some of the paneling on the limbs to really sell the damaged aesthetic should you choose to go that way, I had almost the exact same issue with my MG Zaku J-Type and that's what I did