Game hacks are sometimes really cool, and sometimes they are ho hum. For the most part, the stuff changed is graphical substitutions. Game hacks like the ones for Mega Man 1 and 2 and Castlevania change some of the fundamental challenge of the game while Dr. Luigi falls more in the substitutions realm with the majority of the game unchanged.
Dr. Luigi changes a lot of the graphics, for one Mario is now Luigi (duh). His sprite is based on the 80’s and 90’s versions to stay close to the source game as possible. The viri that are represented are based on the Wii U release with a special bonus if you beat level 20 on the hardest setting.
Since Dr. Mario is not freeware, we cannot link directly to the game but we can link to the patch file on ROMHacking.net