The revs an engine is doing isn't related directly to the power they are making - the 20% referred to is power output not revs. An unloaded engine doing redline is not making as much power as the same engine at 50% of revs being thrashed up a long hill in top gear so the load on the engine is more relevant.
Aircraft engines are (or should be!) designed to handle running at 75% power for long periods of time. I think Rotax have got the formula right while Jabiru have undercooked the engine in a couple of places. So yes, the design of an aircraft engine is different to a motorcycle engine though the materials used are similar.