This collum reads like a social commentary on the demise of a certain size engine not an answer to the poster's question. If the engine size disappears so what? Some company will find a way to get .61 displacement into a .40 size case just lile they did with .50-.52s.
I am hearing that you as a group are say that .61 engines are not powerful enough for these crossover .40-.60 size planes. I have done tests on my thrust bench pulling 9.4 lbs of thrust from a 6" pitch prop from multiple OS 61 FX engines. The K&B Twister pulls well over 10 lbs static thrust on the same props. It seems like 10# of thrust is more than enough for a 6.5# airplane.
If it is flown on the wing, it can be expected to fly with a .40. If you want more try a .46-.50. The K&B weighs the same as most .46 size engines while providing more sport flying power than any of them. Last I knew the twister was selling at bargin prices, at least competetive to the tower units.