Maine actually has a free weekend when non-residents can ride their ATV with no Maine registration, provided the ATV is registered in another state or in Canada. Non-residents may purchase a Maine registration for a week for $53 or for the season for $68. They must do so in person at a registration agent (located in sporting goods stores or convenience stores throughout the state) or at the Maine Department of Inland Fisheries and Wildlife in Augusta.
Check out these Knox, ME ATV/OHV resources: