Capacitor Size for a DC motor. Do I even need one?
If it's running OK then I wouldn't mess with it.

Figure 1. Phase-angle AC control. Image source: LEDnique.com.
Your SCR control is most likely controlling voltage by changing the phase angle as shown in Figure 1. Adding any capacitance will tend to raise the DC after the bridge rectifier on longer than intended and would actually mess up the control depending on the amount of capacitance you add. In addition the SCR on turn-on would have to charge the capacitor on each half-cycle so it and the diodes would be stressed more.
Remember that the inertia of the motor is high enough that it doesn't respond in the &lt; 10 ms half-wave (50 Hz) and it effectively smooths out the pulses of energy it receives giving a smooth rotation of the shaft.