I'm with Roger on this, but I'd add it's not a good idea to keep using it in its present state: each spark removes a bit of copper from the commutator pads, and if you keep at it, you'll score them quite badly. It might be worth replacing the suppression capacitor(s) at the same time, as those failing is another cause of excessive sparking.
If the motor dismantles easily, you can clean the commutator with a bit of white spirit (to lift off the carbon dust), followed by Dura-Glit and kitchen paper to polish it, followed with meths on a pad to get the last of the Dura-Glit off. I use a lolly stick to clean gently between the commutator pads too, if they're not flush with the insulation. Getting any residual carbon crud out from the gaps helps. But you have to be gentle -- the winding wires are quite fine and easily damaged where they're attached to the pads. Don't use anything metal or abrasive on it - at least nothing more abrasive than Dura-Glit, certainly not anything like Brasso or paste. Meths can dissolve enamel too, so don't soak any parts of the rotor or let the pad linger.
It'll still spark a bit with new pads too, for a short while as the brushes have to bed-in to match the shape of the (worn) commutator. It should settle down after a bit though.
