The best I can see through the dirt the stop switch is located in the circled area. The orange arrows indicates the two surfaces that must contact. If these are contacting and it still not stopping the engine you can take an ohms meter and check that switch contact is actually being grounded. If it is being ground then either connecting wire to coil is broken or the coil itself is bad. You can test if the coil is bad by connecting a good length wire the coil's stop terminal and ground it when the engine is running; it should stop it; otherwise, replace the coil. Just make sure you keep away from the moving parts.
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