To the best of my knowledge only the very early engines had a smaller diameter crank pin, actually the very first engines had 1 7/8 mains and two piece main bearing housings borrowed from the parent engine, the model L gas, oops petrol😉 engine. All were recalled best I have read. Past the early versions, which you do not have. A 5/1 only differs from a 6/1 in the operating RPM and HP rating the crank, bearings and rod are same part number. Good luck with your gib keys, my current shop project took me about 20 hours to remove one key and flywheel die to a botched attempt to cut the key out with a torch. If the decide to drill the key as Gary suggested it is best to make a drill guide which is a short piece of the proper sized key with a hole drilled thought it exactly on center and square.