the longsku is regular wordpress post meta, so you can simply call that with get_post_meta.
get_post_meta takes three arguments:
- post_id (which is already available in $_product)]
- meta key (‘longsku’ in your case)
- and a boolean to return a single string (true). if not set, it returns an array, but since the meta key is just a single value you can use true
this should do the trick for your longsku:
echo get_post_meta($_product->id,'longsku',true);