ACF removes it because it seriously impacts the performance of the editor.
You can enable it by adding the following to functions.php
add_filter('remove_wp_meta_box', '__return_false', 20);
There is no tutorials or information regarding moving other types of custom fields to ACF. There have been plenty of discussions here in the past.
Basically, if the custom field holds a single, simple text value then all you really need to do is create an ACF field with the same field name (meta_key). If the field can contain multiple values or is more complex than a simple text value then the fields need to be converted to what ACF needs. The details of every type of field are too complex to go into every possibility. It requires understanding how the existing field is stored in the DB and converting the value to what ACF needs stored in the DB for each type of field.
