I created a basic spreadsheet cross-reference for MAME’s keyboard inputs to correlate to the code for the Arduino sketch. You can view the xls file here.
I have also modified the original Wayne & Layne Arduino sketch that they provided, which I uploaded to Pastebin and is available here. There is extra code that I left in place that they used for LEDs. At this point I won’t be using them but the code remains intact just in case.