restrict-products.php

<?php

function ramdin_the_content( $content )
{
    if ( is_user_logged_in() )
        return $content;
        
    if ( 'product' == get_post_type() )
    {
        $content = 'Please <a href="' . esc_url( site_url( '/wp-login.php' ) ) . '">login to view this product</a>.';
    }
    
    return $content;
}

add_filter( 'the_content', 'ramdin_the_content' );