The migration from Magento 1 to OpenMage is an open-source project to fix security vulnerabilities and update the system PHP version. It’s maintained by the community and completely free.
When migrating a store from Magento 1 (1.9.x) to OpenMage, it can occur that some parts of the store stop working because of the change on PHP version, from 5 to 7, since many plugins and themes are not compatible with the newest version and are discontinued by their developers.
The identification of such problems is usually made by a developer, with the help of Magento debug. Although most of PHP related problems can be visualized whenever the page throws a status 500 error.
To help with these problems, a document describing the mainly problems that happen in the migration and how to solve them. The plugin PayPal Plus, frequently used by brazilian merchants is one of the plugins affected by the migration and described in the document. Furthermore, I make available the fixed version on my GitHub, which can be accessed in the footer of this paragraph. The completed version containing all plugins can be solicited by email.