Does the model balance correctly with the existing engine? Is there any nose weight on the plane to get it balanced? There is a reason for these questions.

According to the Horizon web site, the Saito FA-100 weighs 20.9 ounces including muffler
The Saito Large Singles manual says the 120 weighs 820 grams (about 29 ounces) and I think that does not include the muffler.

So, you'll be adding about 10 ounces of engine weight to the nose of the plane. If there is no weight you can remove, you will increase the total weight of the model by 10 ounces. If you cannot move the battery pack far enough toward the tail to offset the increased nose weight you will have to add even more weight to the tail of the plane to get it balanced.