2.1 - product in admin order details should not be deleted when quantity set to zero

Sometimes, I need to set quantity to zero in admin part when a product is out of stock but I want (and the consumer wants)   keep the original ordered product.

To do that I replaced in orderController.cs this part :

            if (quantity > 0)
            {
                orderProductVariant.UnitPriceInclTax = unitPriceInclTax;
                orderProductVariant.UnitPriceExclTax = unitPriceExclTax;
                orderProductVariant.Quantity = quantity;
            }
            else
            {
                _orderService.DeleteOrderProductVariant(orderProductVariant);
            }

by this one

                orderProductVariant.UnitPriceInclTax = unitPriceInclTax;
                orderProductVariant.UnitPriceExclTax = unitPriceExclTax;
                orderProductVariant.Quantity = quantity;
                if (quantity > 0)
                {
                    orderProductVariant.DiscountAmountInclTax = discountInclTax;
                    orderProductVariant.DiscountAmountExclTax = discountExclTax;
                    orderProductVariant.PriceInclTax = priceInclTax;
                    orderProductVariant.PriceExclTax = priceExclTax;
                }
                else
                {
                    orderProductVariant.DiscountAmountInclTax = 0;
                    orderProductVariant.DiscountAmountExclTax = 0;
                    orderProductVariant.PriceInclTax = 0;
                    orderProductVariant.PriceExclTax = 0;
                }

I would like to change the order total too but I can't find where. Is anybody agree with me and can help me to make the changes.