Every fastener on the Cummins 5.9 and 6.7 engines have been metric for years. Hard to tell from that pic, but I'd bet it's M6 x 1.0. Do you know the length of the stocker? If you have a metric bolt in that size and thread pitch would be easy to determine if the threads are correct. If it is that size, once you determine how long it has to be, metric bolts are available at just about every hardware store.
Now it could be M8 x1.25, but I doubt it. Just about every smaller fastener on the two Cummins I've had were M6 x 1.0 of various lengths.