All that is required to fuel these engines, basically, is liquid nitrogen. We are surrounded by the stuff (most of the Earth’s atmosphere is nitrogen), and making liquid nitrogen is pretty straightforward and is almost a side effect of ‘more useful’ processes. I would expect almost no ‘carbon footprint’ from this, and a spill from an accident should be almost a non-event — even if it gets on you you would probably take less damage than a similar amount of gasoline, if the gasoline catches fire. Even accidentally dumped in an environmentally-sensitive area such as a breeding stream it would take a very large amount to have any particular negative impact (unlike gasoline).
I’d really like to see what comes of this. This appears to have some huge potential.