RE: Upgrade for a OS Max 60FP
For propeller, an 11 x 7, 11 x 8, or 12 x 6 would be good ones to start with, and go from there. See how the engine runs and the airplane flies. Prop it to run in the high 11,000's or better when leaned to just rich of peak RPM while on the ground.
As far as the carb is concerned, OS didn't really make any others that fit. The small venturi is needed to give good handling as well as a good fuel draw. Go to a larger intake venturi, and you'll lose some of the very nice handling characteristics.