I agree with britbrat, it's not critical, so just try to eyeball it straight.

I haven't looked at the Superstar's firewall, but many planes have a little angle built in to the firewall. The idea being that you mount the engine straight on the mount, and the firewall provides the correct right and down thrust for the plane.

So, when in doubt, mount the engine as striaght to the engine mount as you can get it, and figure that the ARF builders probibly got the angle ok on the firewall.

If the angle is off, it will show up in the air. On a trainer, it won't be all that noticeable. Generally, if the thrust line is off to the right or left, the plane will pull that way more than it should when you apply power (note that all planes will pull a little bit as you apply power on take-off). You'll also see a tendancy to pull the nose to the side when you pull from level flight to a vertical climb. Since you won't be doing this much (intentionally anyway) when learning to fly, it's not that important.