But are there actually 851 drivers? There might be some gaps, enough to reduce the total number of drivers to 754. If that's the case, just bump it to 755.
This emulates arcade games, hacks, and multigames that for some reason or another have not made it to MAME. >> Get it at the official release thread HERE.
It sounds like a 851-member array is defined somewhere, you will need to find it and increase the size. Or maybe there is another place you need to add a bit of code. That's my total guess as I don't have the source code.