To answer your question, read the previous post. Better yet, look back through the thread at all the competitions the Venus II has won.
I have a Saito 150 in mine and it fly's great. I am sure the Saito 125 would do as well as I had to add several ounces to the tail to balance the 150 that would not be needed for the 125.
The OS 120 does a wonderful job with this plane.
Save your $$$$$ and fly a great aircraft and it's on sale at Tower now for $249.00 plus another $50 off over $250.00 order.!!!!