Most engine manufacturers configure engines to meet the specific needs of the party who orders them. Kohler offers many different crankshafts. Some are tapered, some are internally splined while other have a straight shaft that is keyed.
The ONLY way to properly make this engine work for you is to obtain the correct crankshaft and install it. The output stub cannot have any runout when rotating or problems will result. Most likely, that engine drove a stack of hydraulic pumps that looked after the sweeper brushes and hydraulic cylinders that controlled the positioning of the brushes.
IMO..... trying to convert this engine by adapting the output stub would be a mistake. Others are free to opine differently.