Lots of blue sparks is a sure sign of a bad armature. You will also need to take a look at the field for damage such as melted insulation. The armature and possibly the field will need to be replaced. The armature is part #5165615 and the field is #636076-7. Both are available from http://ereplacementparts.com.
remove brushes and inspect the copper colored segments. they should be bright, clean and smooth. if it is clean check the brushes and make sure they move all the way in and out smoothly and do not get stuck. check the contact surface area for a nice smooth ware pattern. check the bearings for any play. if armature smells then it is probably burnt up.