Just found this - a copy of "Auto Test", circa 1975. Topic - MG Midget.
"Open sports cars always suffer in performance at the top end when they run with the hood (top) down, and the Midget was no exception. Lowering the hood took the maximum speed down to 94 MPH."
The data sheet indicated a best top speed of 102 mph @ 6,200 RPM, about 700 RPM past the peak advertised horsepower of 66 at 5,500 RPM (DIN).
Granted, the top they were running was the soft top, but at this point, I'm thinking there's strong circumstantial evidence to go with the top on. It's a difference of ~ 7.8%.
It's still a long way from 122 mph, but it's a step in the right direction.