Technological advancement requires complexity. It's the "trade-off" for better engine perfomance, weight, and economy.
This VW type engine is the perfect example of what "simplicity" gets you. Low power, poor economy, poor tbo and heavy weights.
If you want simplicity, you need to accept the other negative features.
The Rotax (the carb version and somewhat the injected models) I see as semi-advanced engine.
The water cooled heads allow higher compression (better fuel economy and power). Full alloy barrels and the gbox again, advance the performance criteria.
I imagine dual carbs goes into that mix as well.
A modern engine. Well, I don't think there is one.
The turbo and injected 915 would have to be closest I guess....injected, turboed, gbox and water cooled heads.