I had a 12 hp Kohler installed in a 109. It was the only engine other than another 10 hp that fit the tractor mounting bolt pattern. I suspect that the 125 is going to have a different pattern for the engine mounts, possibly different attachment of the driveshaft, carb location/linkage, and a lot of other details. You can probably do it, but it won't just be a drop-in job.