According to the sales blurb and the specs in the manuals, the Saito 100 and 125 share exactly the same mounting dimentions as the OS .91.

It fits the Supplied (.60-1.20) Great Planes mount just fine.

In reality i found that the Saito 125 was about 5mm longer from the mounting holes to the prop nut so i ended up drilling new holes in my mounts 10mm behind the holes i was using with the OS .91. If you haven't drilled your mount yet, then that won't be a problem.

I also found the cowl is a better fit now that the engine is back 5mm from the suggested length.

When both engines are complete with muffler the Saito is 50gm (1.8Oz) heavier but that is not an issue because the Skybolt is tail heavy and needs some noseweight anyway.

The throttle linkage is no problem. You will have to rotate the carb 180 degrees and drill a new hole in the firewall to line up with the throttle arm. You have to do that with any engine install on the Skybolt.

No prop issues but 15" is the largest safe diameter with the standard landing gear so a 15x8 is the best choice. Also, the Saito has an 8x1.25mm metric crankshaft so you'll need a new spinner nut.

I haven't touched the firewall. It was never a problem with the .91 but i will be keeping a close eye on it to make sure the 125 is not ripping it out.

The saito needs quite a bit more cowl cutting than the OS. Both for the Cylinder head and the Muffler. My Saito 125 head is bigger than expected and i've just read that they've upgraded the cylinder to a larger size with more cooling fins.