Having recently purchased an O.S. .91 FS, I can recommend it. Easy break-in and lots of power. The papers say use a 12 X 10 for break-in, So I ran that the first 2 tanks. I've since been using a Bolly 13.5 X 6, and the engine isn't loaded down at all. I've put just a bit over 2 gallons of fuel through it, and haven't had any problems.

I've got the one without a pump. Probably the pumped one would be better if you are putting the fuel tank a long way from the engine. My tank is directly behind the firewall.

Really it depends on HOW you want it to fly and HOW you fly! If your looking for something like scale then i would step up to a YS .91 4 stroke! They have GREAT power and fly that airplane very well! I know a few guys that have the same P-47 with YS .91s and love them!

Now if your more like me and love all out balls to the wall POWER then go with the YS .120! My dad had the same P-47 with a YS .120 and was clocked at 153 MPH! and my P-51 has been gunned at 177 to 171 MPH! with running # 1 2stroke fuel and a 15x8 APC prop! and we have never had a problem with them being nose heavy or anything like that. I've also had a TF P-40E with a YS .120 i'll post some pics of them here this week so you all can seen em and everyone of em have YS .120s

Out of the choices you picked the Saito 100 would be my first and your best bet. A YS 120 IMO is way overkill for a scale plane. I love my Saito 72 and I also have the os 70 and out of these 2 i like the Saito better.

If it comes down to the Saito and the YS you would have much more power with the YS, but then again thats why the YS costs more. I have about all the Saito singles and they are great motors. Sometimes you have to let your pocketbook make the choice, you'll be happy with either one

I own all three you have mentioned........I will put my shoes under the bed of the Saito 100.....Scale and power.
The ys, and the os are excellent.....I would not trade my saito 100 for either of them if I could not replace it.
Doc