As the brushes were creating a lot of sparks, I have cleaned the commutator..
How did you 'clean' it? If you used an abrasive of any kind, such as wet & dry paper you may have damaged the copper segments, and/or left metal debris between the lands, or created 'flats' and burrs. The most you should do is use a solvent to gently lift out any carbon dust.
Re-machining a commutator to compensate for wear is a very skilled job, requiring a first class machinist with specialist knowledge and tools to recut the insulation between coppers to the correct width and depth after accurate and concentric turning. That would cost more than a new motor in such a small size. Most likely you need replacement brushes.
( I assume the armature windings are not damaged from your saying the 'cleaning' worked for a few operations ).