Hi,
An unlucky event turned into good luck for me. My thinking was a 150 Saito would be more then enough power. I bought a used Saito 150 about 2 years ago but never used it. I put it in the Stearman but when I went to start it, it gave me problems. I just had acquired a 180 on a great deal so I changed out the 150 for the 180 and I love it. It does everything I want and more. It has power to spare. It's not fully aerobatic but it does things I didn’t expect. Of course I had the 150 fixed and plan on using on another plane.
With the 125 you should get a good scale flight but you end up adding some much weight to the plane I don’t know if it will do anything else.

Dave Brown or True Turn should have a shaft nut and washer that is close. I ended up using the washer and nut that came with the engine; and to that I added a True Turn shaft washer and adapter so the spinner fits nicely.
Some people cut the shaft down to get a fit but to me that degrades the integrity of the motor. It’s up to you if you want to do that.

One more thing! Make sure you check the incidence on the upper and lower wings, and the horizontal stabilizer. They should all be “0” degrees to each other. My upper wing ended up being 4 ½ degrees positive. Needless to say I had to input every bit of down elevator that I had and it still wanted to climb. Later through my investigation of the problem I found that the incidence varies on this plane from negative to “0” to positive. The crew that built mine must have done a lot of celebrating the day before. So beware!
I also moved the CG forward about 1/4 inch. Like I said it flys great now.
I hope this helps,
Shubova
[8D]