Woocommerce Custom Product Data Fields: Update checkbox field to allow it to be checked by default. https://wordpress.org/plugins/woocommerce-custom-product-data-fields/

class-wc-product-data-fields.php

/**
 * Update the checkbox field whith this one
 */

case 'checkbox':
  $thepostid              = empty( $thepostid ) ? $post->ID : $thepostid;
  $field['class']         = isset( $field['class'] ) ? $field['class'] : 'checkbox';
  $field['wrapper_class'] = isset( $field['wrapper_class'] ) ? $field['wrapper_class'] : '';
  $field['value']         = isset( $options_data[0][$field_id] ) ? $options_data[0][$field_id] : '';
  $field['cbvalue']       = isset( $field['cbvalue'] ) ? $field['cbvalue'] : 'yes';
  $field['name']          = isset( $field['name'] ) ? $field['name'] : $field['id'];
  $field['checked']       = isset( $field['checked'] ) ? $field['checked'] : '';

  $default_is_checked = ( $field['checked'] == true ) ? 'checked="checked"' : '';

  echo '<p class="form-field ' . esc_attr( $field['id'] ) . '_field ' . esc_attr( $field['wrapper_class'] ) . '"><label for="' . esc_attr( $field['id'] ) . '">' . wp_kses_post( $field['label'] ) . '</label><input '.$default_is_checked.' type="checkbox" class="' . esc_attr( $field['class'] ) . '" name="' . esc_attr( $field['name'] ) . '" id="' . esc_attr( $field['id'] ) . '" value="' . esc_attr( $field['cbvalue'] ) . '" ' . checked( $field['value'], $field['cbvalue'], false ) . ' /> ';

  if ( ! empty( $field['description'] ) ) {

    if ( isset( $field['desc_tip'] ) && false !== $field['desc_tip'] ) {
      echo '<img class="help_tip" data-tip="' . esc_attr( $field['description'] ) . '" src="' . esc_url( WC()->plugin_url() ) . '/assets/images/help.png" height="16" width="16" />';
    } else {
      echo '<span class="description">' . wp_kses_post( $field['description'] ) . '</span>';
    }

  }

echo '</p>';
break;