I messed with an EPROM dump from my stock chip for a while. You can download WinOLS as a 30 day demo version or something. I was using it for a month. Other projects called though, and honestly, it is a fair amount of work just figuring out where things are on the chip. Once you find maps and actually learn what parameters are what (and how they are scaled), you need to find all the related checksums and modify them according to map changes. The M42 chip is an extra pain...it has all the program code on it, not just maps. The Siemens 80C535 microprocessor in the Motronic only executes the code on the external EPROM module (no onboard program memory), so you have EVERYTHING to wade through. I even tried disassembling the code, but it was a cluttered mess.
In the end, editing the stock chip is probably going to take as long or longer than doing a Megasquirt. Had my first car not been my DD when I was doing MS on it, I probably could have had it all done in a month. Back then, I knew next to nothing of electronics beyond basic analog electronic principles (resistors, caps, inductors, etc). Good luck on this!