I'm using an irvine .72 (equipped with hatori copy tuned pipe), that engine is in the lower end of the scale you would want to use. My plane turned out light (lighter than the instructions said, approx. 3.4kg if memory serves right). The engine has enough power, but I'd buy a os 91 fx if I were you. (Note that the OS fits to my personal preferances, I'm not saying that there is a lack of other decent, or in fact very good, engines available.) A webra 120 sounds awfully heavy in my ears. For the .90 sized engines, APC 14x10 is a very good choice, as earlier mentioned.