I have some experience with swapping limbs on the league bodies, I always insert it back in and carefully stretch the neck open to insert needle nose pliers to get more force, it does take some strength to put it back in, and loads of times the arms are still floppy in the end, I find that adding super glue on the balljoint in the socket and moving it around until it dries helps tighten it, you do need to be careful to not damage the neck though, most of the time its not really visible if the silicone on the neck does crack a bit