Insurance varies around the world, but here are some general suggestions:
You can probably buy physical damage insurance for the motorcycle from the company from which you are renting it. This will cover the motorcycle against physical damage.
Many credit cards include the equivalent physical damage coverage if you rent a vehicle using the card. Check with your card issuer(s). Ensure that the US is covered territory and that motorcycles are eligible vehicles, if you do have such coverage.
Third party liability insurance for bodily injury and property damage is trickier. By law, rental companies only need to give you the legal minimum coverage. Your UK insurance may follow you when you borrow or rent vehicles, so you may want to inquire. You may also be able to add on coverage. However, there may be territorial restrictions that limit what countries are covered. If the US is not included in your policy, you may want to see if a personal umbrella policy is available. This is an additional third party liability policy that sits atop your local automobile and residential policies, topping up the liability coverage with a higher limit and filling in gaps in coverage. Not all such policies cover non-owned automobile exposure abroad, but some do. If you lived in Canada, where I live, and needed the reverse solution, that is what I would do for you (I'm an insurance broker).