You said it's a Saito OS 1.20. Is it a Saito 1.20 or an O.S. Max 1.20? If it's an O.S. Max 1.20, is it a 2-stroke or 4-stroke?

Generally speaking, to go from a 2 bladed prop to a 3 bladed prop, you drop 1" in diameter but go up 1" in pitch. A 14x10 3 bladed prop would be similar in performance to a 15x9 2 bladed prop.

You may want to experiment with different brands and configurations of props in this general size range to find which will work best with your particular engine.