Quoted from vid1900:If you have the ROM that fixes the bonus bug, you can set the dip switch 25 and dip switch 26 to "ON" and then freeplay is enabled.
If you have the factory ROM, you will need to burn a new one:
http://www.pinball4you.ch/okaegi/pro_soft.html
Sorry to resurrect an old thread, but is this still doable? If so, can someone post exactly what I need with detailed instructions. I would like to learn how to do this but currently don't have anything, so I'd be starting fresh.