Protect Content in Custom Fields?

I think you can achieve this by modifying the template. First, you need to determine if the person viewing the post has purchased the associated product, or if the post is not protected. Here is a somewhat messy example…

$is_purchased = false;
$str_products_payperpost = get_post_meta( get_the_ID(), 'woocommerce_ppp_product_id', true );
$current_user = wp_get_current_user();
$arr_ids_products = explode( ",", $str_products_payperpost );
foreach ( $arr_ids_products as $id ) {
  $purchased = wc_customer_bought_product( $current_user->user_email, $current_user->ID, $id );
  // if purchased or if not proteted, show the page
  if ( $purchased || $id == '' ) {
    $is_purchased = true;
  }
}

Then you would wrap the custom fields in a conditional statement so they only show when appropriate. Something like this..

<?php if( $is_purchased ){ ?>
  <div class="protected-custom-field">
    <!-- Your field here -->
  </div>
<?php } ?>

Another thing you can do is render the fields into the editor. ACF has shortcode I believe for that.