M20-Cs were built with a carbureted O-360A1D engine of 180 hp with two magnetos. The IO-360 A3B6D is a fuel injected, dual-magneto 200 hp engine originally used in M20-Js. A few have been put into other models, but the engine STC is required, along with the more expensive engine, different cowling, different spinner and likely different engine mounts and propellor--not something done on a whim. Make sure the airplane logs clearly explain the work done, and that the STCs are present and logged to the aircraft serial number.
Even then, overhaul will be much higher cost than an O-360, even without the frequent conversion: dual magneto to two magnetos.
Safety Note: dual magnetos can cause complete engine failure if one side fails the wrong way.
Experience Note: I have lost my left (cranking) magneto in flight, away from home, and was able to easily land without undue concern. This may or may not be possible when one side of a dual magneto fails, depending on the failure mode.