If you live where there are no emissions tests, remember that you can get the RFID chip from another car's keys, it's SKREEM and ECM - and you are good to go.
Not always a CHEAP way out, but it's a way out.

I still would think that the companies that make "racing ECMs" could produce an ECM that would run an M112 engine and be plug-and-play. Perhaps with a million SLKs on the road, such a thing will happen.
or, maybe not. In any event, this is going to be an issue with all aging cars of the modern age. On the other hand, the outlawing of gasoline-engine cars is not that far off, anyway.