Wayne, you are not going to find an electric motor that will clamp onto the same mount as the YS. 15" prop is not going to be possible with available motors, there just aren't any in the correct KV rating. If you go to RC groups there is an appropriate motor and ESC for sale in the sailplane classifieds. That will require at least an 18" prop. My suggestion if you go with an electric conversion is to ditch the retracts and go with fixed gear.
https://www.rcgroups.com/forums/show...your-BIG-plane