I only know the diameter of the prop for the .455 TPS engine, and that one is 9". It is pretty much a dog for a prop, and one blade was significantly out of balance with the other two. The only advantage that I saw was it was good for braking action when on approach to land. Well, it was good for ground clearance too.

The .61 is a pretty good engine. These engines idle fine and the flywheel weight just bogs it down. The engine will wake up a lot when you remove it, and take out the OS 8 plug. Try both the Saito SS and OS A3 plugs. The A3 is hotter, and may well be the best plug for your engine. The plane will need to be rebalanced when you remove that heavy counterweight.

I suggest that you replace the 3 bladed prop.

The sluggishness is a combination of the counterweight, OS 8 plug (if that is what is in there), and the 3 bladed prop.