Ok, need more help.

My options are : OS .91FX or the Saito 100 (weighs 3.5oz. less than the .91FX.)
Which one is best considering the weight and balance of the aiplane? Or should I go with a Saito.72 which is 7oz. less than a .61 FX?

Basically, I want to select an appropriate engine that will not require adding any additional weight (for a normal CG), front or back.

Thanks