You'd be better off making the pcb remote than trying to shield it. Far easier to eliminate any potential issue completely rather than applying a band aid. This of course assumes that the proximity of the pcb and the motor cause any issues.
To be honest I'd assume proximity of motor to stylus is a greater issue than motor/pcb given that the cart is effectively a very sensitive magnetic field sniffer. Certainly removing a Valhalla from my old lp12 improved the noise measured from the cart output. Placing the Valhalla directly under the motor had no adverse noise effect. Seemed like a worthwhile thing to check at the time.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m