Hello!
I run one of these OS FS. 40 20 years ago ,it was a fine performer, As a matter of fact I still have one of these new in box, never run.
At sea level were I fly I used 5% nitro and 20% castor oil (of course an OS F plug) and an 11x6 APC prop. If you fly at sea level I would think the 11x5 is way to small for this engine. I used my Os in low winged airplane designed by myself with a span of 160 cm weighting 1,8kg and I could nearly do vertical takeoffs using the above combo.

Regards!
Jan K
Sweden