The FL 70 I had was fine in a large trainer. It was not as powerful as the 70 Surpass but it was quite a bit lighter and light is generally good. Yes it is a simpler cheaper engine but mine ran fine and would be fine in the Cub you are talking about. But then again, so would a Saito 56, 62 or even 72. I am really enjoying my 56 and 62 at the moment. The 56 is about 6 years old and going strong and the 62 a couple of months old and getting better all the time. And if you actually need some weight up the front, the old (and rather heavy) Saito 65 was very popular in the Goldberg Cub which is about the same size as you have.