Get item custom field values in onBeforeCartUpdate

-- HikaShop version -- : 2.6.6
-- Joomla version -- : 3.4.4
-- PHP version -- : 5.6.13
-- Browser(s) name and version -- : safari
-- Error-message(debug-mod must be tuned on) -- : N/A

I have products set up with item custom fields which are then used to create a custom file. I need to access those custom fields as the product is being put in the cart so that the image that is displayed in the cart reflects the item values the customer entered for the product.

so far in a plugin on an onBeforeCartUpdate event function I have:

$productClass = hikashop_get('class.product');
$product = $productClass->get($product_id);
var_dump($product);

which displays a lot of product fields but not the item custom values that the user entered before clicking "Add to Cart".

In my onBeforeCartUpdate function, how do I access the item custom values the customer selected?