Quite often when installing an engine especially a 2 stroke on its side we use a Pitts style muffler which puts the exhaust straight out the bottom,so the only extra side weight is the head of the motor.
To counter act that I normally place my flight battery on the opposite side in the fuse to the head.
Of course as you progress to better models and learn more about fine tuning the set up,you will always laterally balance your plane by first adjusting components inside the fuse and then by adding weight if you have to.
Most modelers just trim it out in flight with a bit of aileron or rudder.
Some times on my fun fly planes I balance them by the prop shaft and by the tail wheel assembly with some fishing line looped around each one.
I just add weight to the outer wing tip to get them level and even a large screw inserted into the very end of the wing will do the trick.
Have fun , hope this helps.