Usually the hardware stores carry metric bolts and lock nuts also. I would get the same size bolt and lock nuts. Does the 5/16 have any play? If not they would be ok. The nut retainers I would imagine that some others are the same size but not all. With new lock nuts and some Locktight I wouldnt be worried about the nuts coming lose but thats just me. I would check them once in a while. The bikes make less HP than most riding mowers so I wouldnt worry about the bolts being non factory. Again thats just my opinion. For instance a 400-500hp small block chevy only has 3 3/8 inch bolts holding the torque converter to the flywheel. If that can hold together while moving a 4,000lb car into the high. 12 second 1/4 mile then im not worried about 5 or 6 decent size bolts holding 12-14hp from a machine weighing 500 lbs or less with most riders.