Who said anything about ballooning?

The Senior is an excellent flying aircraft. As you mentioned the right engine! Balance without added balast is the key to a light aircraft with years of use ahead. The Saito 72 weighs 16.5910 Oz's, the ASP 65 weigh's 23.125 Oz's, OS 91 weighs 24.250 Oz. (These are 4 Stroke)

Your 2 stroke: OS 65 LA weighs 18.9 Oz's. With the weight difference you would have had to add wieght.

There is way's to set it up with a lighter motor without using balast. But that would require a C-cell Rcvr pack under the tank and using mini BB servo's on the control surfaces (i.e. Hitec 225BB style servo's) Instead of full size servo's and a larger weight disp Engine. With the size of the control surfaces on this aircraft. I personally wouldn't want to trust a recovery to something that small.