I used Saito 72 four stroke on mine... Others have recommended everything from an OS .61 to a Saito 100...

There are several build threads here on RCU, just no one ever addressed what they did about the CG...

The only tips I have are to:

1. Use thin CA on everything you can get to since the airframe is put together with White Glue

2. Reinforce the firewall with an additional layer of thin aircraft quality ply, pin it and epoxy over the top to seal it

3. Use two servos on the elevators instead of one with the torque bar

4. Put flying wires on the horizontal / vertical stab to keep the tail from "departing" during flight

The last two items are what pushed the CG back on my airplane, but others have indicated that they have done the same to reinforce the tail section and they wrote that airplane flew well... Do a search in the forums for "Passion 3D" and you will find the other build threads.

Mark