Upgrading from Magento 1.9.1.1 to Magento 2.x is possible, but it’s not a direct upgrade. Magento 1 extensions (including the ones you’re currently using) are not automatically compatible with Magento 2. You’ll need to check if Magento 2 versions are available. For example, many vendors like FMEextensions provide upgraded Magento 2 extensions that can replace your existing ones.

Since you’ve also modified core files, those changes cannot be migrated directly. Instead, they should be re-implemented in Magento 2 using proper development practices (custom modules, plugins, etc.).

Themes also need to be rebuilt because Magento 1 themes don’t work in Magento 2. However, your products, customers, and order data can be migrated using Magento’s Data Migration Tool.