If you want truly unlimited vertical then the OS61 series will still be too big but will do the job well but now you are into mounting and balance and weight issuesprevio. As I said previously, the OS50SX will give you that but if you really want power in light weight package then the YS63 is the choice. The power output on 30% nitro is the same as a Saito 100 and the near perfect prop is the APC13X6. If you use 20% nitro you are looking at the power output of a Saito 91

I have one of these in a Great PLanes Cap580 40 size that weighs 7 lbs even and it will hover at 1/4 throttle and go balastic out of that hover. Now you may think to yourself I'll just get the Saito 100 because its cheaper. Well then you are getting into the ability to mount it to the airframe let alone balance it. The YS will be easy to mount, fits in the same mounts as any 46 size engine out there but you will pay allot for this engine and it will be finicky until you get it broke in and dialed it but once you do you'll never go back to another 4 stroke. I own several YS engines and all make gobs of power