I used to be a Kohler dealer a few years back. The 25HP and the 27Hp Kohlers can be interchanged on many types of machines with ease. however, it is doubtful that you will notice any power difference in the two engines. About the only difference I could ever see, was that the 27Hp used a little more fuel /hour. so, 25 vs 27? About the same, go with whichever you can get the best deal on, if you happen to be buying an engine for a repower project.
Jeff