I am not an electronics engineer, but a mechanical one. Yet my understanding is broad enough to be able to read electronic explanations.
The Walbro magnet CDI ignition provides leads on how to go about the issue, and keep the ignition at idle advance until you flip the switch. You need an electronics expert to desighn the system for you, and steer the ignition internal computer to achieve what you want. Some juggling will be needed to get the idle advance just right to limit your rpm to the desired value.
I also am inclined to think that there are better ways to do it, but this comes very close to answering your question.