The white plastic bottle in the picture is the coolant reservoir, which holds the overflow of coolant from thermal expansion/contraction out of the radiator system. The large cylinder in the foreground is the oil tank. Unlike traditional aircraft engines, the Rotax does not have a sump below the engine. Instead, a hose carries discharged oil to this tank. The tank removes air from the oil/foam discharge and provides a reservoir as an oil supply to the engine.
Coolant Reservoir & Oil Tank