It should do well with an OS 70. However, the engine will be a tight fit. I had one with a Saito 72, and the engine was almost touching the firewall. The Saito 72 is slightly shorter than the OS 70 and lighter. Therefore, you will have a tougher time mounting the OS 70. You will also need to add more lead to the tail to balance the plane. The retracts that come with the plane are OK - they work. The plane is a pretty good flyer, but don't expect spectacular performance out of it with the OS 70 since the plane weighs a little bit more than seven pounds and has a relatively thick wing.