That could be said about the first "spy" balloon if anyone had asked the question. At 68,000 feet, a controllable propulsion system on a 200 ft. balloon with a 2,000 pound payload would be quite remarkable due to the insufficient level of oxygen or atmosphere.
The big balloon had propellers — no reason wide chord, thick airfoils can’t work at that altitude. They work on Mars!
Liquid Oxygen, LOX is used. Not simple but easily done.
Carry the Oxygen with you.