Hi Cyberwolf,
I love the O.S. 1.60 FX engines, but it may be overkill on the Edge 540 if it only weights 9 pounds. I have one on a H9 80" Cap 232 that is about 13 lbs and one on a 25% GP Extra 300 that is about 12.5 lbs. Both will hover at about 60% throttle with an APC 18X6W prop. I think I'm getting about 18 lbs of thrust out of them. I would have thought your YS 120 should hover a 9 lb airplane with some to spare? I'm with you, I hate adding weight to balance a plane.
Cheers,
Seth