Yes you can do it, just find the file where the text resides, maybe single.php or single-{post_type}.php or archive-{post_type}.php Then, look for the string you want to change erase it and set a variable, something like this:
If the file where the string is, is a template file for that specific post type, the you will only replace the string for whatever you want:
<h2>Comments are closed<h2> //Replace comments are closed for Hello World
Remember the strings you find in comments, footer or sidebar are in separate files, and, in that case, and in the case the file that is giving the HTML structure to the post type is single.php, you’ll have to do something like this inside the loop:
<?php
$post_type = get_post_type( get_the_ID() );
$the_string = "comments are closed";
if ($post_type === "my_post_type") {
$the_string = "Hello World!";
}
?>
<h2><?php echo $the_string; ?></h2>