I'm guessing too but here's why I'm guessing this way. A KE pass is the same as a slip. You use a slip to slow a plane for landing. A plane that can barely hover looses speed in a long KE pass. At 40lbs a 150size motor would be needed to maintain a KE pass. At 35lbs a 100 cc class engine should do the trick.
It would be interesting to know if a fullsize top of the line aerobatic airplane can hold a KE pass indefinately. I am pretty sure they couldn't just a few years ago. Not untill they were allowed to hop up the IO 540 over about 330 HP.