Hi Rajul.
Just done a very similar transplant, different plane 64" 80's pattern model, but similar engine swop.
I replaced a 91FX (absolute junk) with An OS91FS non pumped (cracking motor), the mounting holes are identical on all three engines 61FX,91FX and 91fs. Another interesting point I actually weighed the 91's and the four stroke is about 3oz lighter than the two strokes if you include the silencers, which is because if you read the OS specs they quote the weights without silencer and the one for the two strokes weighs about 7oz.
Model fly's much better on the FS with a 13x8 master prop.
Cheers futaba1024.