First question -do you have the PTS version [sold as a trainer] or do you have the larger H-9 P-51? The PTS would be great with the Saito 82 or 91. Both are much lighter and have more power than the os versions [OS and Saito are both fine engines but OS's are heavier]. Weight would be the biggest influence on my decision. OS46 and Saito 82/91 about the same weight.
If you have the larger P-51 then either of the 91's, or the 100 would work fine. The Saito 82 would have the power but you would probably be adding nose weight. I fly the H-9 AT-6 with an 82 because that is what I had to use at the time and it does fine. [APC 14x6 9850rpm on 15%]. I was going to put a 100 on it but it flys so well I may just leave it alone.
Fred