The Saito 100 on a 15x4 APC is will beat the OS 91 for power --which is where you are in trouble.
BUT---The OS91 will run on lots of nitro and should be propped for 10000 rpm for best power - you need to open the stinger on the stock muffler as much as possible
We went thru this engine years back and found that basically it is a terrific design -which is seldom used at max power setup
So my own advice is to open the muffler --buy/borrow 30-40% Heli fuel - get a 14 x4 (aprox) prop and try again
Also ditch all purty up weight such as spinner wheelpants -use a good 800 ma high amp battery -etc.