It might even be relatively easy.
I think there are only two ways how the arrow keys could be hardcoded into the exe:
Using Windows scan codes (raw input):
bd850e39-fed1-4cc4-a260-e413fdde32ef-grafik.png
If the right most hex values were found in the exe in close proximity (0xE04D, ...), they could be exchanged with the corresponding values for WASD (see link).
Using Windows Virtual-Key Codes
1b73a278-1d43-4de2-ae88-ccbba14730b2-grafik.png
So in this case these hex values would be hardcoded in the exe in close proximity (0x25, ...), and again could be exchanged (see link, W=0x57 for example).
So somebody would just have to use a hex editor to search for a set of these, replace them, and check if it works.