If you want to do the quick fix, then you only need to modify core.cpp, by substituting paintScreen() code from the the Arduboy library.
If you want to port the game to use the Arduboy2 library, you would delete the local files: ArduboyLowMem.h, ArduboyLowMem.cpp, core.h, core.cpp and glcdfont.c. You would then follow the instructions in the Migrating a sketch from Arduboy library V1.1 to Arduboy2 section of the Arduboy2 library documentation main page, but where it refers to the Arduboy library and class, substitute ArduboyLowMem.
You changed the wrong function, and you didn’t actually replace any code.
You have to change the entire function code for: void ArduboyCore::paintScreen(unsigned char image)
from line 228 to line 244, with the code from the latest Arduboy library core.cpp line 228 to line 250.
The distorted screen is caused by a better compiler optimisation which results in display writes being to fast. it can be easily fixed by replacing all occurances of mul zero_reg, zero_reg with LPM in core.cpp
I’ve also submitted an issue on GitHub.
A forked version with the fix applied can be grabbed here: