Enough oil for at least 1 oil change and anticipated consumption. Oil filter cartridge spares. Spare fan belts. Hydrometer, battery filler, baking soda. Batteries for flashlights, RDF, etc.
Hose clamps, assorted nuts, bolts and screws.
Volt-Ohmmeter and test leads, alligator clips. Tool kit - biggest and best you can afford. Gasket material and adhesives. Spare fuel pump. Spare ignition switch.