I just finished this kit last year. I have had a few of these and have used the OS 40 4 stroke and it flys very well. Not a power house but well. The first kit was built using the stock firewall location. I then switched to the surpass versions with even better performance. All of the latter builds I have moved the firewall back so that the motor would not stick out so much. I also clipped a bay from each wing to shorten the wingspan. Nice plane. I think you will be fine either way. Without moving the firewall you will need to ballance by moving weight to the rear of the plane.

JEB