Outdoor Power Equipment and Small Engines - Start issue w/12.5 hp Briggs lawn rider

I have a Craftsman lawn tractor with a Briggs 12.5 hp engine, 5 speed, 38" cut, Model 917.256451. I cranked the mower up as usual but quickly noticed there was lawn furniture to be moved, so I shut it down before putting it in gear. When I came back to crank again, I heard only a distinct and loud buzzing sound and no start; the buzzing seems to be coming from the engine area. I've checked all the obvious... belts, fuze, battery, plug, etc. Any clue what might be causing this problem when the mower was obviously doing well just immediately before? Thanks for your ideas.

If it's more of a whirring/buzzing, then I'd guess the starter gear has broken or stuck and removal of the starter cover would reveal that.

If it's a rapid clicking type buzz, then I'd say low battery.

The battery checked out and it's strong. I've looked at the diagram, and it looks as if the starter armature might sit in that starter gear. Can I assume that gear can be replaced or unstuck without buying a whole new starter? Is this a job I might do?... Although inexperienced (and poor), I'm learning -- and I'm pretty good at "R and R," following drawings, etc.? God Bless You too!