Hello,
There is few differences between the HikaMarket order status notification email and the HikaShop order status notification one.
I can see that there was some bug fixed and some improvements made in the HikaShop email ; elements which need to be duplicate in HikaMarket.
Regarding the product code, the problem is related to a wrong configuration keyname ; so in the preload you need to replace
'ORDER_PRODUCT_CODE' => (bool)$shopConfig->get('show_product_code', false),
By
'ORDER_PRODUCT_CODE' => (bool)$shopConfig->get('show_code', false),
And the product code will be displayed.
Regarding the custom fields, you're right, you need to add that code into the HikaMarket preload to have access to the "CUSTOMFIELD_VALUE" tag like in the HikaShop email.
I will add that code within HikaMarket for the next release ; in the mean time, since you can create override for email, you can also add the code in the preload and then, use the tag.
With the future release, since the code will be there, you will be able to delete the override for the "preload" (except you made other modifications), but you could also keep it if you do not need the other modifications.
Regards,