Well, you're going to lose the nitrogen one way or another - even if the "car" is insulated, it must be boiling away fairly rapidly in there. (In my experience if you fill a styrofoam cup, for example, at room temperature with LN2, the latter will all evaporate in the space of a few minutes.) I'm just suggesting directing the venting vapor so that it does some useful work
Unfortunately equipment capable of liquefying nitrogen (or otherwise achieve sufficiently cold temperatures that superconductors, even "high-temperature" ones such as the material in use in this demo, will superconduct) is far too heavy to be carried on that little car so a "closed" system (even if you could figure out a way to power it) is out of the question at this toy scale. Still, I bet if they designed it properly they could get the car to zoom around on its own "steam" for at least a few minutes on one charge of LN2.