Way back when I was taught that with altitude and internal combustion piston motors with superchargers, the power available to the prop drops off with density... but the power required to run the motor and the blower stays the same. When power required to run at all is all the motor can put out, that's the altitude limit, and for such motors, it's around 56,000 feet.
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Changing from gasoline to a oxygen bearing fuel such as hydrazine, and changing the motor lets you run the thing higher.
I believe NASA has a CO2 type hydrazine motor that runs up to or close to 100,000 feet.