It's in the carburetor, most likely located on the left side of the engine behind the air filter, follow the fuel line from the fuel tank...
Please post the model and spec numbers of the engine so we can see what particular Kohler engine you have. should be printed on a shiny silver tag on the engine.
Most likely your oil will also be diluted with Gasoline, so drain it and change it, and the filter to. Do NOT run it until you change the oil and filter..