Instructions on Modifying WPC ROM Images

1) Load the ROM image into a binary editor or ROM programmer.
2) Make any modfications to the ROM image you want to.
3) Set the current checksum located at 0x7FFEE & 0x7FFEF both to zero.
4) Calculate the check sum
5) Set the check sum back into into the ROM image at 0x7FFEE & 0x7FFEF.
6) Subtract the checksum from 0xFFFF.
7) Locate two bytes currently set to 0xFF (hopefully unused.)
8) Set the result from step 6 into these location.
9) Save the image and recalc the check sum. It should match the value you
got from step 4.

Also auf gut deutsch:
Checksum im Rom auf 0 setzten
Datei speichern
Checksum berechnen
Berechnete Checksum von 0xFFFF abziehen
Ergebnis im Rom abspeichern