The part number is 90101-942-000. No longer available from Honda. Some NOS ones on Ebay.
It's a 8x1.25 thread bolt.
That's all that's listed on Partzilla for that bolt. The hub bolt, that holds the wheel to the hub, is the same diameter and 25mm long.
The biggest thing is they're a carriage head bolt, so you'll likely not be able to get them at a local hardware. The original nuts are also self locking, with a flange. If you were planning to use regular bolts, it would complicate things by adding more hardware, because you'd need washers and lockwashers, whereas the stock style you only need the bolt and nut (looks cleaner and certainly easier because you won't need a wrench on both sides).
The story of three wheels and a man...