A little higher or a little lower, no problem, but make sure it is as close to center as possible, and I mean the tip of the engine shaft, not the mount itself, remember that most airplanes call for a couple of degrees of right thrust, in other words, when looking at the aircraft from the front, the motor mount will be a tad to the right of the vertical center line of the firewall. Although I've seen engines pointing at every which direction, they flew good enough to learn on. But try to be as close to center as possible, it will fly better.