For owners of a modded Wii U, you can run a community-developed port based on the Sonic Mania Decompilation .

This is the most important part. To play the game legally and make the port work, you must own a legal copy of Sonic Mania on PC (Steam). You need to copy the Data.rsdk file from your Steam folder and place it on your Wii’s SD card. Why You Won't Find a "Direct" ROM Link