For a crankshaft of 10mm diameter, I'd look into 1144 "stressproof" here in North America, 1/2" diameter. Cuts really well. It has become my go-to steel for shafting. Typically does not need to be hardened after machining (i.e. machine directly to finished size) and is likely suitable for strength and hardness - assuming the engine design either uses a plain bearing or a ball bearing and doesn't call for rollers to run on the crankshaft directly.
Other options are 4130 or 4140 but they are terrible to machine on small lathes in my experience - I could not get a good surface finish, and you'd likely need to machine oversize, case harden and then grind / polish to final dimension. (i.e. a lot more work).
Mike
