Kahn,
Mine weights 40 oz. as shown. Add 7.5 oz. for the EI with battery, 47.5 oz. or 3 pounds. It should swing a 15x8 at 8,000 rpm. How much lead do you have in the nose? This engine with mag will weigh about 3.5 pounds. Does the OS 120 fly it well? If you are replacing lead with engine, the balance should remain the same. If the Mac will get the RPM you need without porting you will be ok. I would use a bigger carb though. Test it on the bench and see how well it does.

Dave