The smallest commercially produced fuel injected engine is the Subaru Robin 7 HP "EX21 Fuel Injection".
I can't find it now, but I remember reading a press release on this engine when it was in development. It was rather complex, requiring an electrical power source, a micro processor, numerous sensors, a pump, a throttle body injector, etc, etc.
Regardless of the number of cylinders, for an electronic fuel injection system, you still have to have "one of each" of all the basic components.
I think that to build and market a commercially viable (i.e. "cheap enough") small single cylinder fuel injected engine is quite an achievement for Subaru Robin. As of yet, I don't know of any other small engine manufacturers who have followed them into this market.