Got the electrics for my conversion now. Turnigy SK 42-50 650Kv, 60A ESC, 12x8 Prop and 4S1P 3000mAh Battery. This setup was tested in a classic 40 trainer, very similar to the boomerang, maybe slightly lighter. It flew with lots of power in reserve and long flight times >10min so I think I am happy enough with that to put it into the Boomerang.
Here is the Boomerang with hatch finished but needs motor installed.
Click on image to enlarge.