FD,

The OS 40FS is a four stroke engine and won't have enough power for the Kaos 40. The 40 specified refers to a 2-stroke engine. If your engine is an OS 40SF (subtle difference), that engine will be fine.

The typical prop used on 2-stroke 40's is a 10x6. The SF might also be happy with a 10x7 but I'd stick to the 6" pitch.

David