How to sharpen my Briggs & Stratton Select Series 4.5 Hoursepower lawn mowe?

Is there a way to sharpen myself the blades on my Briggs & Stratton Select Series 21 in. Cut 4.5 horsepower lawn mower? I also need to replace the plastic cover over the cable that controls the speed at the top on the left side, do you know where I can find this piece? My 13 yr old son has it working at the moment by using duck tape, but I'd like to replace the broken piece if I can.

Thank you, in advance for your help.

Debbie
May 2010