I talked to the Flathead County Treasurer today about getting my vehicle registered in Montana and getting a plate. I had started down the road of registering my business in Montana and getting it titled under the business but I don't think I'll need to.
If I understood correctly a person needs to fill out 2 forms. Form MV1 and MV70. They are printable on this page.
Forms & Manuals: Vehicle Title & Registration - Montana Department of Justice
The page for Flathead county treasurer is
Flathead County Department of Motor Vehicles
You'll need to fill out MV1 and MV70 and also provide proof of ownership. Snail Mail these items to the treasurers office. They will process the paperwork and send you a license plate.
MV70 is the form that validates your machine is street legal. In short you need head lights, tail lights, brake lights, brakes, horn, and at least 1 mirror.
I'll get my paperwork sent asap and update here.