Difficult. Code on traditional BW titles (like Road Show etc.) contain compiled code burned onto ROM chips (Read Only Memory). You literally had to burn new ROM chips to update code (unlike modern Stern, JJP games today). So you would have to have something to first read the code from ROM, then de-compile the code, interpret it, re-write it, burn back etc. Not easy (if not impossible). But today, it's easy to buy a new controller board (like a P-ROC controller from Multimorphic) that you can put into a BW game and write your own custom code from scratch (in a language like Python.) Of course, you're talking thousands of lines of code to match the typical level of game play we see in games today so it's still a heck of a lot of work. But people (including me) are doing it!