I've contacted Technopower to ask if it's possible to make a Stearman PT-17 of Balsa USA with 9 cylinder engine but Marc Linville, president of Technopower Engines said me that:
"Sorry to inform you that our engine is much too small to work with the model you have selected! Ours is a 1/5 scale engine. The maximun recommended weight for our 9C engine is 22 lbs..."
so I think that it's uncorrect to change scale!
Sergio