The only OS FSR engines I'm aware of were the .10 FSR, .25 FSR, .40FSR, .50 FSR, .61 FSR and 1.08 FSR. If it's an older engine, are you sure it's not a .91 FS Surpass?? The FSR models were 2 strokes, and until recently, the only .91 airplane engine OS made was a 4 stroke. There is a current .91 2 stroke, but it's an FX. It could also be (depending on its age) just a regular .91 FS. Can you post a pic or 2?