You can find parts, diagrams, and your owner's manual on www.SearsPartsDirect.com
Your model number is missing the last digit, here are all models that begin with 316.79500: searspartsdirect.com/search?q=31679500
For help with parts, call Sears PartsDirect at 1-800-366-PART.
Hope this helps!
If you need a new model, here are all weedwackers: sears.com/lawn-garden-line-trimmers/b-5000409?sortOption=ORIGINAL_SORT_ORDER&searsTab=true
This is the brushcutter attachment that will fit any gas powered weedwacker above: searspartsdirect.com/product/5ptdgks3pl-0071-247/id-41ajbc-c799