Just saw your post and have a little additional info for you. Hope it's not too late.
1. Each state has different laws regarding registering vehicles.
2. Most states have a sales tax (like VAT), they vary, but 8-9% should be the highest you'd have to pay. Oregon and perhaps Nevada have no sales tax.
3. Generally, in order to register any vehicle and get a license plate/tag/reg number one must pay the sales tax. If you buy from a dealer they will add the tax to the price of the bike. If you buy the bike from an individual then the state will most likely require payment of the sales tax at time of registration.
4. Usually if you buy from a dealer they handle all the interface with the state. You give them the $, they give you the bike and a couple of weeks later you receive the title/registration certificate in the mail. In the interim you will have a temporary registration and/or plate.
5. Therefore, you will need a local mailing address. There are many small businesses (like the UPS Store or Mailboxes Etc.) that rent "post boxes" & addresses for a small fee. They will even forward mail to you as you cross the Continent.
6. Many states also require a mechanical inspection of the vehicle. Some, like California don't (they do road-side safety inspections randomly). Some state require even new vehicles to be inspected. Most just for 2-3 year old vehicles. Motorcycles fall under the same rules as cars, generally.
7. Liability insurance for the motorcycle is required in most states. They are minimum amounts in each state- Progressive, GEICO and other insurers will help you, as will a motorcycle dealer (they get a commission on the insurance cost for referring you). But, you only need to obtain the insurance in the state where the vehicle is registered; all other states accept the registration and insurance. States that require insurance require you to furnish proof of insurance when you register the vehicle. The insurance company will provide a certificate of insurance for you to ride in Canada. However, you must buy separate insurance for Mexico, generally available at the border, Don't go there without it. The border guards may not stop you at the crossing, but they have checkpoints about 100km into the country and will stop you, they could confiscate the bike if you don't have insurance.
This all sounds very complicated, but it really isn't especially in New York, California and other states that have a lot of travelers from other countries. It is not unusual for visitor to buy a vehicle on the East Coast, drive it cross-country and resell it some where along the Pacific Ocean.
May I suggest you contact a dealer in NY, like BMW Manhattan, regarding your interest in buying a motorcycle, explain the situation and ask them how to go about doing it. You shouldn't be obligated to buy from them, but it's a good dealership (as are Irv Seaver-BMW and Brown's in Southern California). Contact BMWMOA.org and they may be able to help you too.
If you are a member of RAC or another national auto association, you may be able to get advice form them and then utilize their colleagues at the American Automobile Association(AAA) in the US to help too.
Anyway, enjoy your ride and ride safe.