Those are notorious for misfire generally at the higher end of the RPM scale approx 5000 rpm. I'd say replace it with a newer dizzy, there are a lot on the market that specialise with this motor as enthusiasts that race them have found that they combat common issues such as cross fire, misfire, burn out, rotor bounce etc.
Aftermarket dizzies can have several advantages;
1. Up to 9000 RPM operation (twice as much as standard)
2. Optimum advance curve
3. Low amperage use
4. Twice the fire over magnetos
5. Improved throttle response
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