Based on the plan on my wall and the size of the OS .91 Surpass the answer is no, the fuse is only 5 inces wide, so the OS 1.20 AX would be a close fit at best. The main problem is getting really good airflow over the cylinder. The radiator opening is vertical and will provide better airflow for vertical (either inverted or upright). Plus invertedgets the prop coming outcloser to the center of the fuselage for a more scale look.

Not to mention that a side mount would possibly getting in the way of the modifications to the sides of the fuselage where I am going to be adding detailed valve covers and header/exhust,