IMO a Saito 125 will not be enough. Maybe a 150 or 180 for sure. Mine has a O.S. 120 AX 2 stroke and it is really good on power. The Cermark Pitts is no 3d plane but really good aerobatics with the 120 ax.
As for the inverted verses side mount my vote is for side mount. I always try to avoid inverted although 4 strokes seem to handle inverted better than 2 strokes. Fuel tank height in relation to the carb is critical in inverted engines.