The specs on the IRF520 are deceptive. @v8dave, I am not too sure what that op-amp is doing but at 12 volts, the gate should be at least 4.5V to get about 1A of drain current. This is nowhere near the “low” gate threshold voltage in the spec! Sorry about that @james211 
So because of this, I would recommend a motor control shield. @Hypnopompia hooked up an Adafruit motor shield v2.