Either the YS110 or the OS 120AX will work well in your YAK. The YS will run well on 15% nitro. The YS has a better midrange transition, the throttle response is instant. With the OS the powerband is more in the upper to high rpm range.
Both engines will pull a 9lb plane like crazy. I have a YS 110S on a 9lb extra 260. The plane hovers at a little less than mid throttle and takes off vertical like a rocket. If you do the research on setting up the needles and regulator than the YS hardly ever needs adjustment. I was running 30% nitro for a while, 10,000rpm on a 16x6APC. I have changed to 20/20 fuel and the RPM dropped to 9800. I could probably run on 15% and still have gobs of power for my size plane.

The quality of the Saito and OS FS are nice, but they just don't have the same power as the other two.