I can't say for sure, but I don't think I could bolt it to the flywheel of the B&S. And, since it is recoil start, it would probably be difficult. I'm I correct in assuming that?
Install the engine with the flywheel in front. Before you hook up the drive shaft, remove the charge pump and turn it upside down. It's labelled LH and RH for rotation. One is for driving the hydro from the engine flywheel and the other for driving from the engine PTO. Don't forget to turn it back when you reinstall the Kohler.
It doesn't take a whole lot of power to do what you plan and 5 hp should do the job nicely considering that the hydro will deliver about 4 hp to the ground. My Husqvarna grass cutter can only deliver about 2.75 hp to the ground and it will tow a 2000 lb trailer with little effort.
Big horsepower numbers are for powered implements. The tractor alone uses only a fraction of the power available for the drive, and 4 horses is a lot of power for the drive. Although fast acceleration won't be one of it's claims to fame, it will pull an impressive load.