Except that a Cessna 150 is certified to CAR 3, not FAR 25 or CS-23.
Having owned one for some 20 years, the Cessna 150's maximum altitude is limited by power, certainly not by Mach.
As an aside, I thought about getting the 150 HP conversion with a service ceiling of 23,000 ft. -- Just think a CE-150 calling center with "XYZ at FL210, requesting higher."
The Goldfish