The Lumia 520 modding scene has slowed but not died. Key resources:
Current unsolved problems:
You will see many tutorials claiming you can install Windows 10 Mobile on a Lumia 520. Nokia Lumia 520 Custom Rom
To install a Custom ROM, you must bypass the secure boot.
Phase 1: The Emergency Flash
Phase 2: Using Nokia Care Suite (For Modified ROMs)
The easiest "custom rom" for the Lumia 520 is not an Android fork, but the leaked Windows 10 Mobile builds. Microsoft officially blocked the 520 from receiving Windows 10 due to the 512MB RAM limit. Developers bypassed this using a Custom ROM image (.ffu file). The Lumia 520 modding scene has slowed but not died
If you are looking to modify your Lumia 520, you are essentially looking for modified versions of the official Nokia firmware. These modifications usually unlock features restricted by Microsoft or carriers.