Hand held sparking trick
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Although simple, it's quite tricky to suggest a motor. The rotating wheel will present a load on the motor and will require torque to start, the flint will also act like a brake and the starting torque has to overcome this as well.
The motor you are using, is it geared? If not you may get away with a lower voltage motor without a gear box. Just a motor like this one https://uk.rs-online.com/web/p/dc-geared-motors/2389642/ may be OK.
Or if the speed from this motor is suitable I'm sure it would start with the load you have and works down to 1.5V (3V max) https://uk.rs-online.com/web/p/dc-geared-motors/4130593/
You may also find cheap motors by searching the web for high torque model aircraft motors, quadcopter motors or hobby motors which may not be as professional as the above, but good enough. If you have any old toys with a motor these may also be good enough to test and alow them to recycled.