Hey Lukerman, Some of the Bosch distributors had an optional drop-in rotor available that installs a simple and inexpensive mechanical rev-limiter. As the engine RPM climbs, a calibrated spring gradually stretches through centrifugal force to direct the ignition spark to ground at a predetermined point. As the RPM drops, the ignition spark returns to the plugs.
Although I have not seen one of these things for a 928 - it may be available as a special order. You could then pop in the rev-limiting rotor just before your daughter drives your 928. As an additional safety precaution just stretch the spring with a few good tugs to artificially lower the cut out point to around 1800 RPM. That should put her top speed somewhere around 55 mph…..
Guess I should have looked at the year of the vehicle being discussed - the rev-limit rotor would more than likely not be an option on an 86
Good Luck, Michael