What is the fastest unpressurized aircraft with oxygen masks that can carry people?
Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor.
The operational ceiling was limited by what the pilot could endure for several minutes while breathing oxygen from a mask, without losing consciousness. Pilots underwent altitude chamber training to harden them.
Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.
The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h.
About 370 were built.
In the height of 16000 feet, a human can survive on normal atmospheric pressure. Above 16000 oxygen level keep getting down and changes on pressure. So we are talking about the aircraft which flies till 16000 feet. Cirrus SR22, Beechcraft Bonanza, Cessna 172, Cessna 182 Skylane, Cessna 310 are some fastest unpressurized aircraft.