OP, you do realize that the Genesis essentially contains a Master System, right? The Power Base Converter is a glorified pin adapter, with no CPU or anything else. I don't know whether Sega Master System Brawl runs in SMS or Genesis mode, but all the hardware is already present to run anything for either system (minus a few oddball SMS games), with no conversion needed.
The number of systems that can meaningfully run each other's code without porting from scratch is quite small. Outside of backwards-compatible systems like the Atari 7800 and Genesis, there's the Intellivision, which can run an interpreted form of RCA Studio II code with major intervention (the reasons are complicated); the ColecoVision and Sega SG-1000, which have near-identical architectures, but still require significant rewriting for any given title; and one or two other similar examples, including some amazing work done on NES > PC Engine (TurboGrafx-16) ports.
But that's it. Any other conversion, especially between consoles of the same generation, will require a rewrite from scratch -- as much work as making the game in the first place, if not more (since you don't have source code, etc.).