Replace fishing rod with custom entity

I'm trying to make a grappling hook, but need it to be a custom entity to preserve velocity and stuff. I have the custom entity set up, I just need to replace it when the player launches it. I currently have a PlayerFishEvent set up to handle grappling.

Code:java

public void onPlayerFishingEvent(PlayerFishEvent event){
        Player p = event.getPlayer();
        Material item = p.getItemInHand().getType();
        if(item == Material.FISHING_ROD){
            java.util.List<Entity> nearby = p.getNearbyEntities(50,50,50); // searches in a 100*100*100 radius around the player for other entities
            Entity hook = null; // holds the future hook
            for (Entity e : nearby) { // loop through entities
 
                if (e.getType() == EntityType.FISHING_HOOK && ((Player) ((Fish) e).getShooter()).getName().equals(event.getPlayer().getName())) { // it is a hook!
                    hook = e;
                    break;
                }
            }
            if (hook != null && isGrappleValid(((Fish) hook))) {
                Location hookLocation = hook.getLocation(); // the location of the hook
                Vector vector = hookLocation.toVector().subtract(p.getLocation().toVector());
                p.setVelocity(vector.multiply(0.3));
            }
            else {
 
            }
        }
    }