I guess you could 'Make' somthing to steer the stones away with a few off cuts of an old shell and some double sided tape and form some cleverly.
One thing id wach out for would be stones near youre spur gear (the mean drive gear) if youre on loose dirt / stone it would be very easy for one to chip a tooth or get caught causing damage.
It wouldnt be that hard to create a spur gear cover
