Black & Decker KS850 Circular Saw pinning sporadically
Looks like it might need new carbon brushes, or maybe the motor commutator is worn out.

If the plastic piece that wasn't important to you was a part of what holds the brushes, rotor, armature, commutator, or any motor pieces in place then you could have mis-alignment of any of the above that might cause sparking like that. Take the motor housing apart and look closely for crooked stuff, there should be some blue/black marks from that sparking to clue you in.

P.S.- unplug the saw first