I have been using this corded lawnmower for four years. Satisfied with no emission, lower maintenance, quietness and lightness. Allowed someone to use it, and now the motor slows and blows a house fuse after a few minutes. Upon inspection, the fan has a pie wedge shape missing. Further inspection, (removal of the shroud and switching back on) sparks can be seen just below the cap end. Does this mean that the entire motor has to be replaced?

That motor appears to have a commutator and brushes, meaning that there will be arcing between the commutator and brushes. This is normal.

Motor is probably over heating from damage fan.

Replace fan, #29 on below parts list below and you should be fine.

Black and Decker LM175 Parts List and Diagram : eReplacementParts.com (http://www.ereplacementparts.com/black-and-decker-lm175-electric-mower-parts-c-4167_9514_9548.html)

Hi,
An excellent link; one that I would have given had I not been beat to it. And, prices are right there. Motor is extremely pricey, but fan is not bad nor are the brushes, which are held in by #59 and #6. Here is a link to the pdf version:
http://www.ereplacementparts.com/images/black_and_decker/LM175.pdf
For nicer printing, but no prices.
Peace,
Clarke