How to display a custom post as a page (but unedited)?

To use page.php specifically, you can use the template_include filter.

You could also copy page.php in your theme, saving it as single-foo.php, and it would then be used for your “foo” CPT’s individual posts. You would want to do this in a custom or child theme rather than editing a parent theme directly, so that whenever your theme is updated, you don’t lose this edit.