If the OS 90 is a four stroke then any of those engines will work with varying levels of performance. A 4 stroke would be my choice as it can swing a prop that looks like it fits that plane and the sound is ubeatable. The ST 45 would be a good choice if you have fairly light winds in your area and are just learning to fly. The Evo 61 would be about on par with a 90'is four stoke without the sound.