RE: Welcome to Club SAITO !
The main difference between the two engine is the external size.
The 115 has the same mounting footprint as the Saito 65, 80, 91, OS/Magnum 70.
The 125 has the same footprint as the Saito 100, OS/Magnum 91, Y-S 91,110, & 115
There is one prop size difference in power between a 115 and a 125.
The 115 is essentially a Saito 91 with a 125 cylinder on it.